Test Your Mechanical Aptitute

October 18, 2007 9:27 PM

Are you fairly handy? Well how about theoretically handy? Take this test to find out how mechanically apt you are.

Hint: be very literal when looking at the pictures and read the questions carefully. Don't over-think. (ie, "bean-plate"). Click on the symbol at lower left to show the question list, which will show you what answered correctly or incorrectly. When you're finished, you can click on any missed question to discover the correct answer.

posted by maxwelton (112 comments total)

I'm a pretty fair shade-tree mechanic, electrician, etc. and scored an 84, mainly due to a lack of block-and-tackle learnin'.
posted by maxwelton at 9:31 PM on October 18, 2007

It took me nearly a minute to answer the first three questions. I'll save some time and just admit that I have trouble operating a hammer.
posted by dhammond at 9:41 PM on October 18, 2007

If gear A is turning at 400 rotations per minute, how many questions will it take before I click the "x" on this tab in Firefox?

Handiness. Pffft. Whatever. Hands are for typing.
posted by katillathehun at 9:44 PM on October 18, 2007 [4 favorites]

94%. Maybe I should stop being a programmer and operate a block & tackle for a living.
posted by aubilenon at 9:47 PM on October 18, 2007 [1 favorite]

Went too fast on a couple...82%...

That was fun.

Is the piston intake one right? I thought suction was the force at work.
posted by Exchequer at 9:54 PM on October 18, 2007

Ack. 78%. I actually enjoy some handyman-type activities, but I know I'm not mechanically inclined naturally. My brother, on the other hand, wowed my dad and uncle when he was about a first-grader by correctly figuring out the pipe sequence for the water softener.

I only did as well as I did by thinking through the physics. Very few of the answers came naturally. And many of them I realized were wrong the moment I had the next screen up!
posted by dhartung at 9:59 PM on October 18, 2007

78% here, though I didn't check my answers until I was done which I'm sure caused me to miss some of the questions in series.

The business about "Doesn't test knowledge, just aptitude" is completely bogus. About a third of the questions required specific knowledge of one type or another.
posted by tkolar at 10:00 PM on October 18, 2007

I am, theoretically, 68% handy. Which comes as no suprise to me.
posted by brain cloud at 10:02 PM on October 18, 2007

Hmm...I consider myself very mechanically inclined. I do my own car repairs, am the son of a heavy duty mechanic (who taught me everything from small engine repair to the clutches on bulldozers), have a gift for spatial relationships (I can always pack things into less space than others), and work in a very logical field (system administration).

I only made 78%. Clearly I'm better at real life than quizzes.
posted by Kickstart70 at 10:03 PM on October 18, 2007

74% -- not bad for an English major.

Block and tackle was a breeze after spending a few years around square-riggers. Gear assemblies posed some challenges, and I wasn't sure how to calculate the PSI on the large-cylinder to small-one problem.

Cool test! Thanks.
posted by Miko at 10:11 PM on October 18, 2007

96%. Now I don't feel too bad for only getting 40 on the rice game.
posted by zsazsa at 10:19 PM on October 18, 2007

There was definitely some knowledge type questions that i didn't have a clue about (ok perhaps a small clue after I saw them all), the electrical circuits ones for example.
posted by edgeways at 10:19 PM on October 18, 2007

The business about "Doesn't test knowledge, just aptitude" is completely bogus. About a third of the questions required specific knowledge of one type or another.

Yeah - I blew my score on not knowing the names of different types of drive/cam/crankshaft, and not knowing the electrical symbols and the like. Phooey. (And yeah, embarrassingly bad on block and tackle force calculations.) But I did well with the rest, so go me. Fun quiz.
posted by LobsterMitten at 10:21 PM on October 18, 2007

I'm wondering about the piston one too. The whole idea of turbo and superchargers is to compress the air-fuel mixture going into the cylinder, right? Do engines without those compress the air fuel mixture to some extent before it goes in? If not, how does it get into the cylinder?

Or better, how is the correct answer of atmospheric pressure pushing it in differ with the incorrect answer of it being sucked in by the piston traveling down?
posted by Andrew Brinton at 10:24 PM on October 18, 2007

94%. Very fun test, especially when you haven't played around with this stuff in ages.

Also, I agree about the suction/piston question being misleading. And the fact that this very much is a test focussing on knowledge as opposed to aptitude, at least with regard to the "label each electric symbol" bit.
posted by Sticherbeast at 10:28 PM on October 18, 2007

92%. Missed the Piston question too. In retrospect, suction is sort of semantically incorrect. When you suck on a straw, for example, the atmospheric pressure is pushing on the surface of the drink and driving it into your mouth. It's a meaningful distinction if you are approaching engine performance analytically. Otherwise, you're not wrong, you're just not right.
posted by Carmody'sPrize at 10:33 PM on October 18, 2007

94%.
Exchequer: Piston creates a vacuum. Atmospheric pressure fills the vacuum. Without the atmospheric pressure, nothing happens.
posted by weapons-grade pandemonium at 10:37 PM on October 18, 2007

100%, but I cheated by having a degree in mechanical engineering. Some of the terminology based questions definitely required specific knowledge rather than aptitude - like the naming of types of gear.

The piston air intake was a dubious question. I'd suggest both 'suction' and 'atmospheric pressure' are correct. Suction happens because of a difference in pressure caused by a change in volume. It's a semantic distinction and anyone answering either way understands what's happening in principle.
posted by normy at 10:38 PM on October 18, 2007

In addition, no turbo, no additional compression, other than the compression of about 14.7 psi that we walk around in every day. You can think of the turbo as a device which lowers your effective altitude to well below sea level.
posted by Carmody'sPrize at 10:40 PM on October 18, 2007

92%. A few of the questions were bullshit, or at least badly written: for example, the question about balancing boxes didn't make it clear where the center of mass of the boxes were, and the question about the pistons gave forces in PSI (not a unit of force).

This is an excellent post, by the way. I think this kind of knowledge should be part of everyone's basic technical literacy, just as people should be expected to have read some classic novels. We're living in a world where references to Shakespeare plays and the Bible are common, but mention the second law of thermodynamics in polite company and you'll get blank stares!
posted by Bletch at 10:40 PM on October 18, 2007

Some of the question were metric, and some such as the piston question asked for imperial measurements.

I don't do imperial
posted by mattoxic at 10:45 PM on October 18, 2007

96%, I forgot what the symbol for a fuse was. Also, that suction/air pressure question was totally bogus.
posted by [expletive deleted] at 10:53 PM on October 18, 2007

90% not very good for a mechanical engineer. the piston intake question is bad because the piston moving down causes a low pressure in the cylinder which makes the now relatively higher atmospheric pressure to push the air into the cylinder. so really the answer is both because they're the same thing, but i picked the wrong one.

there was also a see-saw question that didn't place the cg of the boxes at their centers, thus thwarting me because they're wasn't a 1/5 of 300 lbs on the list. and no-one said the balloons weren't coexisting next to each other. oh well
posted by TheJoven at 10:55 PM on October 18, 2007

Oh, and for those who were flummoxed by the pulley questions, the trick is to figure out the ratio between distance pulled and the distance the weight on the other end travels. If it's 1:1, then forces are 1:1, if the distance is 2:1, then the forces are 1:2, etc.
posted by [expletive deleted] at 10:55 PM on October 18, 2007

94%, which completely surprised me since I don't know how to do anything under the hood of a car beyond pour in oil and replace the battery, and I haven't done physics since grade 9...
posted by Jimbob at 10:57 PM on October 18, 2007

Yeah, I guessed the answer I thought they wanted for the see-saw, rather than what I calculated based on center of gravity - bit of a lame diagram. And I agree, the balloons picture could have been a lot better.
posted by normy at 11:00 PM on October 18, 2007

84% overall. Specific knowledge is required for many questions. The question about whether it was 'an overdrive or a reduction gear' was just dumb, because you couldn't tell where the engine and the wheels were, so you couldn't tell which way the force was going.

One question came up as incorrectly answered even though I didn't get that question on the test. The question that I supposdly answered 'wrong' had two weights balanced at dissimilar distances from a central point, and it wanted to know how much weight B had to be. I think the question I actually got was the piston pressure one, with the large upper plate and the small plunger.

The only one, in my opinion, that I genuinely missed was the pipe with A and B chimneys; I got it exactly backwards for relative heights.

I don't even know how to change oil.
posted by Malor at 11:01 PM on October 18, 2007

Rocked it. Here's a question for you.

Steam system. You use boiler steam to preheat feed water. At what point does the steam lose more energy than feed water receives?

(Hint: Think pressure)
posted by Mblue at 11:07 PM on October 18, 2007

“Is the piston intake one right? I thought suction was the force at work.”

Yeah, what Carmody'sPrize said, except it's not semantically incorrect, it's just plain incorrect.

I got 94%. I made a stupid mistake on question 7 (overdrive, equal, reverse, reduction), I should have known the physics of 44 but don't (which open tube will water go higher up), and 50 which was a nomenclature question (camshaft, driveshaft, etc) that I just guessed at.

I was annoyed when the aptitude stuff changed to things requiring nomenclature and such.

“Do engines without those compress the air fuel mixture to some extent before it goes in? If not, how does it get into the cylinder?”

When the piston increases the volume in the chamber, the atmospheric pressure outside pushes the air/fuel mixture in. It is not compressed. However, the piston in its reciprocating motion starts to decrease the volume of the chamber again, compressing the fuel/air mixture, and at the minimal it's ignited. In a gasoline engine this is done with a spark plug. In a diesel engine the compression itself causes the mixture to ignite.

“...and I wasn't sure how to calculate the PSI on the large-cylinder to small-one problem.”

The surface area of the large side of the piston was seven time greater than the area of the small side. So the PSI on the smaller surface was seven times greater than that of the larger.

“Or better, how is the correct answer of atmospheric pressure pushing it in differ with the incorrect answer of it being sucked in by the piston traveling down?”

Again, it's not true that the universe doesn't like a vacuum. There is no such thing as "suction". There's only a greater pressure pushing “in”. I'm surprised that auto mechanics are expected to know this. “Naturally aspirated” just means that the fuel mixture isn't forced into the chamber mechanically.

I like to explain the “there's no such thing as suction” by saying that not even Superman could drink water from a straw more than 40 feet in height (on Earth) because what's making the liquid go from the cup into your mouth is the weight of the column of air above the cup to the top of the atmosphere. That's equivalent to about 33 feet of water at sea level, if I recall correctly. Anyway, the point is that Superman could suck as hard as he wanted and that water isn't going to go any higher than about 33 feet. Suction isn't a real mechanical force.

I remember taking some kind of mechanical aptitude test like this with gears and pulleys and such in seventh grade and scoring abnormally high. I think the test also involved spatial reasoning and mental rotation of objects.

“there was also a see-saw question that didn't place the cg of the boxes at their centers, thus thwarting me because they're wasn't a 1/5 of 300 lbs on the list.”

Hmm? One box was three times the distance from the center of the see-saw than the other; meaning that if they were balanced then one box weighed 300lb and the other 100lb, if I remember the question correctly.

“The question about whether it was 'an overdrive or a reduction gear' was just dumb, because you couldn't tell where the engine and the wheels were, so you couldn't tell which way the force was going.”

I got that one wrong, too, but on review I realized that by process of elimination it could only be answered one way. Put another way, the question and its possible answers implied how the gears related to the engine and wheels. Specifically, the possibilities were direct, reverse, overdrive, and reduction. Three of the gear combinations caused rotation in one direction, the other in the opposite. So you could tell which one must be reverse.
posted by Ethereal Bligh at 11:08 PM on October 18, 2007 [2 favorites]

84%. You know what? Fuck electricity.
posted by cortex at 11:09 PM on October 18, 2007 [1 favorite]

I see what you guys are saying about the see-saw and boxes. I saw the problem as they saw it. But now I think they are wrong. If those boxes are balanced, the right box really must be one-fifth as heavy as the left box, not one-third. The proper measurement is distance between the center of the pivot (or whatever the correct term is) and the centers of gravity of the boxes, not box lengths.
posted by Ethereal Bligh at 11:17 PM on October 18, 2007

Hmm. I am 80% handy, which means I'm 20% thumbs. That's a ratio I can be comfortable with.
posted by mumkin at 11:18 PM on October 18, 2007 [2 favorites]

Got one wrong. Probably one of the several that were vaguely worded.

The uneven seesaw one bothered me because I think they were looking for a three-to-one ratio, but the way things were drawn I think the real answer is actually closer to five-to-one (looking at the center of mass of each weight rather than its outside extent.) Thankfully, they didn't give 60kg as a possible answer or I'd have been stuck.

And in the two-fans-facing-each-other one, I know which way the other fan would turn, but whether that's considered the same or opposite direction depends on whether you use each fan's front as a reference point or a common reference point for both.

Not sure I want somebody who can't express themselves clearly teaching MY diesel techicians.
posted by Opposite George at 11:18 PM on October 18, 2007

Ethereal Bligh-
the center of the boxes were 1 'unit' away on one side and 5 'units' away on the other side. the overall length of the sides was 2 and 6, what they wanted you to see, but I bean-plated it.
posted by TheJoven at 11:21 PM on October 18, 2007

84% but I failed the part where I was supposed to find out which questions I missed. I'm fairly sure I blew the planetary gears question.
posted by bz at 11:22 PM on October 18, 2007

need to learn to use the preview button...
posted by TheJoven at 11:23 PM on October 18, 2007

On reading everybody else's gripes, let me underscore my position that EB is totally correct.

And, to clarify -- on the fan one, the problem is whether you use a single point in space or two -- each relative to a corresponding point on one of the fans -- to determine fan "direction." Obviously I have no business teaching diesel technicians either.
posted by Opposite George at 11:23 PM on October 18, 2007

88%, because of piston suckage, a bernoulli blowout, and a
seesaw screwup. That seesaw was 1:5, not 1:3, and anyway
the dog wanted out just then, so ...
posted by the Real Dan at 11:24 PM on October 18, 2007

“And in the two-fans-facing-each-other one, I know which way the other fan would turn, but whether that's considered the same or opposite direction depends on whether you use each fan's front as a reference point or a common reference point for both.”

I don't think that was ambiguous. As a word question, it is. But in the real world, or the world of your imagination, what matters is the relative motion of the fan blades to each other, not to the face of the respective fans. If you walked into a room with these two fans in this scenario (facing each other, one turned on) you'd almost certain say that the fans are “turning in the same direction”.

“the overall length of the sides was 2 and 6, what they wanted you to see, but I bean-plated it.”

Yeah, but don't you think they are wrong? It's not the length of the sides relative to the center that matter, only the centers of gravity relative to the center. That rightward box has to weigh 60lb, not 100lb. You can know this is true from experience (not theory) by thinking about how a traditional balancing scale works.
posted by Ethereal Bligh at 11:31 PM on October 18, 2007

If you walked into a room with these two fans in this scenario (facing each other, one turned on) you'd almost certain say that the fans are “turning in the same direction”.

Yeah, that's what I put as the answer, but in the real world of learning about fluid couplings -- which is where they probably drag this example out -- the single reference point is handy when picturing torque transmission but not so hot when thinking about how things work when you're staring either mating part in the face. It's especially germane when you have to think about the appropriate handedness for a stator clutch.

Bean-plating or not, five or six words would've resolved the ambiguity.
posted by Opposite George at 11:47 PM on October 18, 2007

72% and I'm an extremely drunk biologist.

I wish that they broke down wrong answers and described why your answers were wrong. I'm sure I got answers wrong because they were on a screen instead of in front of me.

I think that this is why so many people are anti-academic. They Everyone has had bad experiences in grade school. When one's wrong, they get ridiculed instead of corrected.

I've been corrected many times, and I'm sure you've all been corrected - I guess the "thing is to instill respect into "formal communication" - how many rich kids lolca5ts when they're writing "Thank you so much Aunt Mimmy for the $44,000 you sent me to commemorate my 22nd birthdday"

xxxooo
posted by porpoise at 11:52 PM on October 18, 2007 [1 favorite]

I should have known the physics of 44 but don't (which open tube will water go higher up)

Bernoilli's Principle. You can avoid all the maths by knowing that as the speed of a fluid flowing down a tube increases (at some constriction, say), its pressure decreases, all else being equal and ignoring all sorts of real-world effects like viscosity, shear and turbulence, among others.

center of the pivot (or whatever the correct term is)

"fulcrum"

Perhaps the boxes have non-uniform density...
posted by normy at 11:54 PM on October 18, 2007

TheJoven writes "there was also a see-saw question that didn't place the cg of the boxes at their centers, thus thwarting me because they're wasn't a 1/5 of 300 lbs on the list. and no-one said the balloons weren't coexisting next to each other. oh well"

Yep. Considering they didn't tell you any mass distribution in the boxes the correct answer should assume uniform distribution which means 1/5 was the ratio.

Malor writes "The question about whether it was 'an overdrive or a reduction gear' was just dumb, because you couldn't tell where the engine and the wheels were, so you couldn't tell which way the force was going."

This tripped me up too.

Malor writes "that I genuinely missed was the pipe with A and B chimneys; I got it exactly backwards for relative heights."

They don't tell you whether the flow is from pressure or suction. The question is indeterminate.

Opposite George writes "And in the two-fans-facing-each-other one, I know which way the other fan would turn, but whether that's considered the same or opposite direction depends on whether you use each fan's front as a reference point or a common reference point for both."

This question is also indeterminate because of lack of information. Which direction the fan turns is strictly determined by the physical properties of the fan blade. We all know that fans do not create vortexes from experience. There for the air flow can be ideally modelled as if the air flow is being generated by blowing on the fan with say compressed air. Because the air is hitting head on whether the fan has a clockwise or counter clockwise blade orientation is going to determine direction.


TheJoven writes "the center of the boxes were 1 'unit' away on one side and 5 'units' away on the other side. the overall length of the sides was 2 and 6, what they wanted you to see, but I bean-plated it."

This only works if the mass is all concentrated at the extreme outer edges of the boxes.

The oil/water question was indeterminate as well, at a minimum some bunker C oils will sink relative to water.
posted by Mitheral at 12:14 AM on October 19, 2007

I scored 60% which means I "did not pass" but I'm feeling pretty damn good about it!
posted by amyms at 12:28 AM on October 19, 2007

72%, I messed up overdrive/reduction (assholes), fans because I used the wrong reference point, and a bunch of pulleys questions because, well, I don't really use pulleys much. I know, this is a saddening hole in my existence. And "suction" is to me an accurate way to describe the process of increasing the volume of a chamber to encourage flow.

I like mechanical things. This was just your average grade seven "pulleys and levers" physics section, and, well, uh. I can't say that I'm too put off by my test failure.

I thought it was going to be things like "blah blah is happening to car. Is it the spark plugs?" Or "what do you figure is wrong if you hear this noise?"

You know, everyday things that might reflect your ability to deal with the average real-world problem, as opposed to the number of pulleys you need to lift a mystery box. Lame.
posted by blacklite at 12:31 AM on October 19, 2007

84%, because I sucked at the block and tackle stuff. Fun post, definitely going to pass this one around.
posted by mosk at 12:51 AM on October 19, 2007

84% because of block and tackle suckage, too. And what is suction if not the creation of a vacuum? Two correct answers to that one.
posted by grubby at 2:30 AM on October 19, 2007

I got 76%, and I'm okay with that because I don't even own a car or any tools except a swiss army knife. And I didn't know that gasoline explodes better than diesel...well, I do now! It was fun, but it definitely tests knowledge, not 'basic talent' - I did most of it remembering high school physics.
posted by jacalata at 2:44 AM on October 19, 2007 [1 favorite]

SUCK SQUEEZE BANG BLOW, ASSHOLE.

Sorry, had to get that out.
posted by IronLizard at 3:22 AM on October 19, 2007

94%, I assumed that question 15 would want me to take pulley friction into account, but I was wrong. I knew I was over-thinking it. The piston and pipe questions indicate I should learn more about fluid dynamics I guess.
posted by BrotherCaine at 3:34 AM on October 19, 2007

Got 86% and I think I should have done better given that I'm an engineering graduate albeit electrical rather than mechanical. Got all the electrical questions correct thankfully or else I would have had to hung my head in shame.

Got tripped up by badly worded questions like the fan one.
posted by electricinca at 3:38 AM on October 19, 2007

I was gonna take this test, but then I fucked up my laptop turning it on and when my car broke down on the way to the computer repair shop I wasn't able to fix that, either. Then I dropped my celphone when I tried to call CAA for roadside assistance.
posted by The Card Cheat at 5:04 AM on October 19, 2007 [1 favorite]

86% and yeah, if the actual scenario was in front of me as opposed to (bad) diagrams thereof, I probably would have scored better. I feel kind of dumb for chosing suction now though. I should know better.
posted by Skorgu at 5:18 AM on October 19, 2007

96%. Both of the ones I missed were fluid-dynamics questions. I've never studied fluid dynamics.

the question about balancing boxes didn't make it clear where the center of mass of the boxes were

What, you don't have point masses lying around at home?
posted by oaf at 5:27 AM on October 19, 2007

I hate gears. I rocked everything but the gears. I need them in front of me so that I can turn them just a little. I was so relieved when it moved to pulleys and almost cheered on the electrical questions.
posted by cobaltnine at 5:48 AM on October 19, 2007

84%
English Grad. student
posted by exlotuseater at 5:55 AM on October 19, 2007

Am I the only one who got annoyed when they asked for force in kilograms?
posted by backseatpilot at 6:10 AM on October 19, 2007

84% again mostly because of miscounting on the block and tackle ma's.

As my advisor used to say, pumps don't suck. It was his favourite way to bollox candidates on their orals. Pumps deplete, then the vacuum fills by diffusion. While the piston motion creates a region of low density, the driving force refilling the cylinder is atmospheric pressure. The pressure imbalance can be described using Fick's second law---diffusion.
posted by bonehead at 6:18 AM on October 19, 2007

Am I the only one who got annoyed when they asked for force in kilograms?

No.
posted by bonehead at 6:18 AM on October 19, 2007

The questions on the order of "if gear A is turning at 400 revolutions per minute, how fast is gear B turning" are horribly imprecise.

From the picture, it looks like gear A may be half the size of gear B; but I could be wrong. Maybe it's 3/7 the size of gear B.

As a mathematics graduate student, imprecision is my bane, and this test is rife with it. I couldn't finish it.
posted by King Bee at 6:41 AM on October 19, 2007

68%... Not good for a mechanic.... but not bad for a programmer. My dad would be mad tho, he is a quality engineer and would say didn't you listen to anything I ever told you about.... at that point I would start day dreaming about chicks or tacos..... tacos rule
posted by Mastercheddaar at 6:45 AM on October 19, 2007

"Theoretically handy" is right. I got 92% because I studied physics before dropping out due to &*^) Al Gore inventing the Internet at just the wrong time, and missed most of what I missed because I refused to sit there and count gear teeth. I can't work a hammer properly and I have to get people in to put Ikea furniture together. And I'm glad I don't have any children, because anything with a chirping "Your parents put it together!" in the commercial would cause severe embarrassment.
posted by darksasami at 6:53 AM on October 19, 2007

King Bee, while the diagrams weren't tremendous for the gear stuff there were exactly ten teeth on the small gear and twenty on the large, so that much at least seemed unambiguous in intent.
posted by cortex at 6:54 AM on October 19, 2007

Glad I wasn't the only one to flag that piston intake question...I want a 2 percent curve on my grade...

For you real automotive / combustion experts: Is it possible in a normally-aspirated engine to get more than "atmospheric" pressure inside a cylinder at the bottom of the stroke due to the velocity of the air rushing in to fill the partial vacuum?

If so, then suction is the true and accurate answer.
posted by Exchequer at 7:11 AM on October 19, 2007

I scored 74. Although this is a failing grade, it is still much too high because physics gives me hives and my mechanical aptitude ended with Lincoln Logs. In any fair test I would have scored a 14. I am enjoying the critiques of particular questions (such as the two fans opposite one another) because they make me feel better about missing them. Did anyone else miss the balloon question by failing to read the first sentence?
posted by A Long and Troublesome Lameness at 7:15 AM on October 19, 2007

#10 is also disputable for me. The gears are at right angles to each other so who's to say which side the second gear's motion is reckoned from? Sure it's shown from a certain side, but should the answer depend on the orientation of the viewer?
posted by StickyCarpet at 7:26 AM on October 19, 2007

Where was the plane on a treadmill question?
posted by yeti at 7:30 AM on October 19, 2007

96%. I clicked the wrong button on one and found you couldn't revise your answers. I had to guess at the direct/reverse/... question but guessed right using reasoning as described above. None of the questions require any units knowledge. If you understand the principles as long as the units are consistent within a problem it's just ratios. In general if you feel there isn't enough information (such as center of gravity) make the simplest assumption. In engineering you do this all the time to bound problems and then fine tune it as information becomes available.
posted by substrate at 7:37 AM on October 19, 2007

I got the fan question wrong because I wasn't sure what "opposite direction meant in this case". I wasn't sure if the direction was relative to an observer or with respect to the front of the fan. I guessed wrong.
posted by substrate at 7:38 AM on October 19, 2007

Where was the plane on a treadmill question?

It was raided by vikings and left in a flaming pile off frame.
posted by cortex at 7:39 AM on October 19, 2007

There is no such thing as "suction". There's only a greater pressure pushing “in”.

When I taught this concept in environmental education classes, we used to say "Nothing in nature sucks."
posted by Miko at 7:42 AM on October 19, 2007

To anybody still arguing about suction, that question on the test was specifically testing your awareness that suction is a false force. If you got it wrong, you got it wrong.
posted by darksasami at 7:51 AM on October 19, 2007

Certainly, force in kilograms isn't strictly correct in any textbook sense, but there's no harm in it for quick back-of-an-envelope calculations. g can be ignored on both sides of an equation, as long as you understand what you're doing. Saves multiplying by 9.8 all over the place only to divide again at the end. "100 Kg force" is sloppy shorthand for "the force applied by the weight of a 100Kg mass at the surface of Earth" - it would be wrong in an article or formal specification, but for quick mental calculations or sketching it doesn't change the outcome if you understand the mechanics.

Similarly, I might as well try to defend sucking, while I'm here. Again, yes, of course everyone's right when they say there's no such thing in a strict physical sense. Of course there's no magical suction force that mysteriously translates matter from one region to another and it's really all about pressure differences. But in a colloquial everyday discourse, I don't see any problem with saying that a piston "sucks in the fuel-air mixture" during the induction stroke. It doesn't demonstrate any less understanding of the basic operation of a four-stroke petrol engine.

Maybe this is just one of those engineers vs. physicists things.
posted by normy at 8:02 AM on October 19, 2007

FAIL. 64%. I'm actually shocked I did that well, honestly. My dad's a diesel engine mechanic, so I must have gotten some of that through osmosis.
posted by desjardins at 8:14 AM on October 19, 2007

The problem with sucking is that it isn't equivalent to negative pressure. If a sucking force existed, Superman could suck up the oceans from orbit, to use Ethereal Bligh's example from above. "Suck" as a force leads to non-physical behaviour. It is therefore a bad model to understand fluid flows.

It matters in engines, because the maximum refill force is 15 psi, which depends on the atmosphere alone, and not at all on how the engine creates the pressure differential. So the right way to think about it is as an external pressure, not as an internal negative one. Draw a free-body diagram, if you have trouble following this argument: all will become clear.
posted by bonehead at 8:27 AM on October 19, 2007

94%. I would never, nor have I ever heard anyone call me a "handy" person. Heh. Get off my couch.
posted by carmina at 8:30 AM on October 19, 2007

I think I have the lowest score posted so far: 54%. There were things on that test I have never even thought about, and certainly never covered in school. (I did catch several errors, though. Remember, the "principal" is your "pal.")

I guess I have some reading to do!
posted by fiercecupcake at 8:35 AM on October 19, 2007

Don't worry, fiercecupcake, I'm sure you don't have the lowest score.
posted by DenOfSizer at 9:03 AM on October 19, 2007

Mech. 1: Hey, we're smart, too! How come we don't get no respect?

Mech.2: They make you take an aptitude test to get into college, right? We need an aptitude test.

All: YEAH!!

And it seems to work.

They are smart-- much smarter than most of the rest us begin to suspect, or than they know themselves, I think, but this test does not even head in the direction of the distinctive sort of intelligence necessary to be a good mechanic, in my opinion. That has much more to do with a capacity for direct sensuous grasp of the world, an uneasy feeling for when something is wrong in a functioning device or system coupled with an instinctive desire to set things right, and a direct physical apprehension of cause and effect in relation to your own body of the sort an athlete or musician also requires. A considerable capacity for abstract thought in a more conventional sense is also necessary, of course, but it's secondary.
posted by jamjam at 9:11 AM on October 19, 2007

Cortex - Ahh. Then I admit that I suck at physics/engineering. =)
posted by King Bee at 9:15 AM on October 19, 2007

It's okay; I do too. The only thing I liked about the Mech. Eng. building on campus was the photo of Tacoma Narrows they had hanging on the wall.
posted by cortex at 9:29 AM on October 19, 2007

Ethereal Bligh wrote...
I like to explain the “there's no such thing as suction” by saying that not even Superman could drink water from a straw more than 40 feet in height (on Earth) because what's making the liquid go from the cup into your mouth is the weight of the column of air above the cup to the top of the atmosphere. That's equivalent to about 33 feet of water at sea level, if I recall correctly. Anyway, the point is that Superman could suck as hard as he wanted and that water isn't going to go any higher than about 33 feet. Suction isn't a real mechanical force.

That's one of the better explanations I've heard for that. Thanks.
posted by tkolar at 9:30 AM on October 19, 2007

Now, Batman on the other hand...
posted by cortex at 9:34 AM on October 19, 2007

I'll admit that I got 52%, just to skew the numbers here. I can open pickle jars without too much trouble.
posted by picea at 9:57 AM on October 19, 2007

Suction may be a false force, but it's a real and useful concept. Otherwise we wouldn't have a word for it. I only accept that atmospheric pressure is the more correct answer because it's the more important variable in engine tuning. I imagine the hypothetical diesel technicians involved would normally take as given the volume of air required at maximum. The intake manifold air pressure, which varies due to many things outside the engine, is of more practical import.
posted by sfenders at 9:59 AM on October 19, 2007

78% for a high school physics teacher who can't change his own oil. I should have gotten an 82%, but I screwed up one of the block and tackle problems and one of the gear problems. On the whole, I'm happy with my results.
posted by Hactar at 10:27 AM on October 19, 2007

“This tripped me up too.”

When I was thinking about it later, I realized that Malor was complaining that you couldn't tell which side of the diagram corresponded to the engine and which side corresponded to the wheels.

But because the question is set up such that you can logically deduce which is the reverse gear (because it's the exception among the four gear diagrams) then if you recall that reverse is a high-torque, low-speed gear, you can tell which side is which in the diagrams. That's a little subtle, though. But it means that you can figure which is which of the overdrive and reduction diagrams.

“They don't tell you whether the flow is from pressure or suction. The question is indeterminate.”

Since the vertical pipes must be open-ended, then if the flow was from “suction”, there would never be any flow up either of the pipes, making the question senseless. Thus, you kow the flow is driven from the left.

“This question is also indeterminate because of lack of information. Which direction the fan turns is strictly determined by the physical properties of the fan blade. We all know that fans do not create vortexes from experience. There for the air flow can be ideally modelled as if the air flow is being generated by blowing on the fan with say compressed air. Because the air is hitting head on whether the fan has a clockwise or counter clockwise blade orientation is going to determine direction.”

No. The design of the fan blades doesn't matter, as long as the two fans are identical, which the question states. The question is asking whether the two fans will rotate the same direction, or opposite directions, relative to you, the observer. It doesn't matter which way the fan blades are designed. You turn on the one powered fan, and the blades will turn in direction A, relative to you (or anything external to the fan). it pushes air against the unpowered fan it's facing, and this will cause that fan to turn opposite to its normal powered motion, which will be the same direction relative to you. In other words, if the top of the left fan is moving toward you, the top of the right fan will be moving toward you, too.

“The oil/water question was indeterminate as well, at a minimum some bunker C oils will sink relative to water.”

That's true, but water is unusually dense for a liquid at STP and most oils at STP are lighter. It's not unreasonable for the question to assess whether you know this bit of practical information.

“The questions on the order of "if gear A is turning at 400 revolutions per minute, how fast is gear B turning" are horribly imprecise.”

As cortex says, you don't need to know the relative diameters of the gears, you only need to count the teeth. The teeth drive the gears, their rotation is determined by them. It's not ambiguous.

But cortext also says, “at least seemed unambiguous in intent” implying that the gear teeth don't physically determine how they will work. But they do. Of course, if the sizing is wrong enough, the gears will jam. But counting the teeth does not give you an approximation, it gives you the exact answer.


“Is it possible in a normally-aspirated engine to get more than ‘atmospheric’ pressure inside a cylinder at the bottom of the stroke due to the velocity of the air rushing in to fill the partial vacuum?

If so, then suction is the true and accurate answer.”

No. This is partly why thinking about “suction” is misleading. The energy causing the filling of that chamber is atmospheric air pressure. How could it cause greater than atmospheric air pressure? You're thinking about the inertia of the moving air, but it's not going to have enough inertia to force it to more than atmospheric pressure. If it could, you'd be able to build a perpetual motion machine with air rushing back and forth between chambers. This is obviously wrong, so your thinking must be confused by the addition of your idea of the “sucking force”, which doesn't exist.

Also, the reason that this question “cares” about it being atmospheric pressure pushing the fuel/air mixture in, is because until the upstroke, the pressure in the chamber is the atmospheric pressure. This determines the energy density of the mixture and that matters to engine design. If it was some arbitrary force “sucking” the air in, then you wouldn't know what pressure the fuel mixture was at before compression.

As bonehead puts it:

“It matters in engines, because the maximum refill force is 15 psi, which depends on the atmosphere alone, and not at all on how the engine creates the pressure differential. So the right way to think about it is as an external pressure, not as an internal negative one.”

”The gears are at right angles to each other so who's to say which side the second gear's motion is reckoned from? Sure it's shown from a certain side, but should the answer depend on the orientation of the viewer?”

You assume that the point of reference for the given rotation of the left gear indicates the point of reference you should use for the rotation of the right gear. It doesn't matter what that point of reference is. Why would you pick different point of reference? This is like asking the same thing about the regular gears where you object because looking at the second gear from behind gives you a different answer.

“Suction may be a false force, but it's a real and useful concept. ”

It's a common concept, but it's misleading. Its utility is limited by where it misleads. This is one of those cases. There are many others.
posted by Ethereal Bligh at 10:48 AM on October 19, 2007 [1 favorite]

Suction may be a false force, but it's a real and useful concept. Otherwise we wouldn't have a word for it.

I'm going to have to say Nuh-uh to this too. We have lots of words for stuff that doesn't make sense: elf, ghost, compassionate conservatism.

This is one of those seemingly-sensible concepts that we learned at six that just doesn't work. There aren't many places in the human experience of the physical world where "common sense" breaks down, but this is one of them. If suction worked, was real and useful, you woun't have had to spend money to put a pump (or a turbo-system) at the bottom of your 30 ft well. Suction doesn't work however, so practicaly every person without a municipal supply has to fork over extra dough to their well contractor.

The way things work sometimes does go against "common sense", and part of being science- or engineering-literate is overcoming these sorts of bumps. This is only a minor one; quantum mechanics is one of the major mind-fucks in any physical science education.
posted by bonehead at 10:49 AM on October 19, 2007

Ethereal Bligh writes "No. The design of the fan blades doesn't matter, as long as the two fans are identical, which the question states. "

No it doesn't. I just whipped through to 38 and the question states:

Two fans sit opposing each other, one is turned on and the other is not on. What do you expect to happen.

The choices are nothing, rotate the same, rotate opposite.

As a further nitpick the diagram shows only one fan plugged into the wall. As they don't specify which is turned on if you choose the one that isn't plugged in then nothing happens.
posted by Mitheral at 11:49 AM on October 19, 2007

MBlue -- If I understood your question correctly, in a steam-boiler-fired engineering plant, like on a ship, I think where the steam loses most of its energy is when it's driving a turbine (or turbogenerator), right before it goes into the condenser to condense from steam back down to recycled feedwater for the boiler -- right?
posted by pax digita at 12:02 PM on October 19, 2007

If suction worked, was real and useful, you woun't have had to spend money to put a pump (or a turbo-system) at the bottom of your 30 ft well.

Only if you misunderstand what "suction" means, as I guess some people do. It's just a handy name we give to a difference in pressure. It's only misleading if you misunderstand it, replacing its proper meaning with some kind of nonsense.

I for one will keep using it anyway, particularly when talking about engine manifold vacuum, since everyone else does, and now that I think about it, it seems there's no real possibility for confusion unless you're talking about some theoretical aspects of forced induction or something. In an engine, unlike your 30-foot well, there's always going to be some air sucked in as long as the ambient pressure is above zero.
posted by sfenders at 12:06 PM on October 19, 2007

Of course, if the sizing is wrong enough, the gears will jam.

Exactly. Or slip. Or skip. Not that any of that looked likely from the diagrams, but you know, hence the qualification. If we're gonna lovingly nitpicking this motherfucker, let's be thorough about it.
posted by cortex at 12:11 PM on October 19, 2007

98%. On one hand, I've never even owned a car or done any maintenance besides adding gas and oil, but on the other hand, I grew up on a farm with plenty of machinery around, plus I have 2 math degrees.
posted by fings at 12:16 PM on October 19, 2007

“The choices are nothing, rotate the same, rotate opposite.”

Well, okay, you're right: the question doesn't make it explicit that the fans are identical. But the diagram implies it and it seems very strange to me that you wouldn't assume the fans are identical.

If the fans are identical, they will rotate the same way.

“As a further nitpick the diagram shows only one fan plugged into the wall. As they don't specify which is turned on if you choose the one that isn't plugged in then nothing happens.”

That's just silly. When you and other people nitpick like this, I think you don't realize the implications of what you're saying. I mean, it's also the case that the fans could exist in some alternate universe with different laws of physics. Or, more mundanely, the fans could have a ratchet mechanism preventing them from rotating opposite to the their powered rotation. When you answer questions like this, you use common sense to evaluate the question as its author intended. If you claim this isn't possible or isn't necessary, then almost all questions are unanswerable.

I do agree, though, that it would have been if the question had made explicit the frame of reference it was assuming for how it was describing the fans' rotation. I think it rightly assumes most people will imagine themselves looking at the fans as if they were sitting on a table in front of the person, the fans appearing as they do in the diagram—and that the frame of reference of rotation is implicitly the person (and not, say, the face of each fan). But they should still have just made that frame of reference explicit.

I left a comment for them about the incorrect box-balancing answer, by the way.

“It's only misleading if you misunderstand it, replacing its proper meaning with some kind of nonsense.”

Most people believe that a vacuum creates its own “pulling” force, causing things to fill it. I can't say whether most mefites believe this because this thread implies otherwise; but I'm certain most of the rest of the world believes this, with a tiny number of people knowing otherwise. So most people do “replace its proper meaning with some kind of nonsense”.

And there are lots of other engine designs that don't use evacuation of the combustion chamber and allowing it to be filled via atmospheric pressure for aspiration—that's why the question specified “naturally aspirated”.
posted by Ethereal Bligh at 1:47 PM on October 19, 2007

Most people believe that a vacuum creates its own “pulling” force, causing things to fill it.

Right, so we'll have to avoid saying "vacuum" as well I suppose, as it can be abused in exactly the same way.

I'll have to remember that next time I make any reference to using a drinking straw to lower the density of air above a laterally-confined column of water so as to allow atmospheric pressure to force a liquid through it against the force of gravity.
posted by sfenders at 3:33 PM on October 19, 2007 [1 favorite]

No, wait, forget water; I'm switching to some other liquid now.
posted by sfenders at 3:39 PM on October 19, 2007

Thanks, Ethereal Bligh, for confirming that the inertia of incoming air cannot create more than atmospheric pressure in a cylinder. I withdraw my objection.

I still want the fucking a 2 percent curve on my grade, though...
posted by Exchequer at 4:51 PM on October 19, 2007

On the Reduction, Reverse, Direct, Overdrive question how can you not tell where the engine is?

On every diagram of the test, the Drive gear is the one with the little curvy arrow wrapped around it.


I don't know what the arrow circle thing is called but it is pretty obvious that it is the one with a motor attached.
posted by Megafly at 5:07 PM on October 19, 2007

68%. There's a reason my main vehicle is a bicycle.
posted by Tuwa at 11:20 AM on October 20, 2007

I got an 84%, which is pretty bad for an aerospace engineering major. I missed one of the simple gear questions by mistake, and a pulley question (the one with several different size pulleys all connected by a rope), and some of the stupid electrical questions.

I wasn't terribly bothered by being asked for\given forces in kilograms, since it is reasonable to assume we are on Earth, and can therefore multiply by 9.81 to get force. What really bothered me was that they called PSI a force, and then asked that question, because the force on the two things is. the. same. It's the pressure that's different.

The sucking questions was stupid. I realized that sucking and being pushed in by the atmosphere were the same damn thing, so I just picked one. If they wanted to test whether you knew that sucking isn't a force or not, they should have come up with a better question.
posted by !Jim at 1:16 AM on October 21, 2007

the inertia of incoming air cannot create more than atmospheric pressure in a cylinder.

That's incorrect, actually. The principle is obvious enough; move anything through the air and you'll compress the air in front of it. I suppose it creates a pressure wave which moves at the speed of sound. That's "air speed", so if the air is moving the opposite way the pressure wave will in fact go slower from a more stationary reference point. The intake manifold attached to the running engine of a car is filled with these pulsating waves of sound, which is why they're designed with resonator chambers to absorb the noise.

Say the piston stroke is 10cm. The speed of sound is 340m/s, so that's 3400 piston-travel-lengths per second. An engine at 7200RPM does 120 complete cycles per second, so in a four-stroke engine I guess that'd be 1/480 seconds for the intake stroke; so in the time it takes for the air pressure to adjust to the new motion of the piston, it would move a significant fraction of the stroke if its speed were constant, which of course it isn't. In fact it is moving only relatively slowly near each end of its range, so I suspect there would be no significant effect either way whether the intake valve closes a fraction of a millisecond before or after the piston reverses direction.

EB: If it could, you'd be able to build a perpetual motion machine with air rushing back and forth between chambers.

Maybe if you had Maxwell's demon to do the work of opening and closing the chambers...
posted by sfenders at 3:48 AM on October 21, 2007

By 1/480 seconds I mean 1/240 for a better guess... not that it makes a difference.
posted by sfenders at 4:01 AM on October 21, 2007

Oh, I guess you wouldn't need Maxwell's demon either. Just a perfectly inelastic and frictionless material, which would no doubt be very useful for all builders of perpetual motion machines.
posted by sfenders at 4:57 AM on October 21, 2007

Okay, so it's more complicated than I thought. Still, I think it comes down to being equivalent to a cylinder closed at both ends under acceleration; the pressure at one end should be higher by a small amount.
posted by sfenders at 7:25 AM on October 21, 2007

“Still, I think it comes down to being equivalent to a cylinder closed at both ends under acceleration; the pressure at one end should be higher by a small amount.”

That's true, but it's not equivalent.
posted by Ethereal Bligh at 6:04 PM on October 21, 2007

Not equivalent, but it did help me understand. Thinking of how the situation is similar makes it clear to me that it's at least possible for air pressure at the piston as it stops to be higher than just what it needs to equalize the pressure.

I also found it helpful to imagine the analagous situation of a canal filling up with water from the sea. It can be filled to above sea level if you close it off just as the wave reflection from the inland end reaches the sea. Having nothing better to do at the moment, I verified this experimentally. Lest anyone be yet not convinced, I report here the results.

I used a beer bottle on its side, partially submerged in water. Black pepper was floating on the water surface so as to observe its motion. The water level inside was the bottle lower than in the sink.[1] It's sealed with my thumb. It's fixed firmly in place. Open it to let water flow in; it does for a while, then some flows back out; then a much lesser amount flows back in again.[2] There is some possibility of a sub-surface current to contend with, but if I close the intake valve at exactly the right moment[3], let the water settle so there's no significant currents, then open it, an easily observable quantity of water flows out.[4] If I do the same when the water levels are equal this does not happen. I conclude that the water level inside the bottle ended up higher than sink level.

This does not violate the laws of thermodynamics, since the difference in water levels is always less than it was originally.





[1] Initially I tried starting with the bottle empty, but this made things far too complicated. Much easier to see what's going on when the water level difference starts out small, about five millimetres.

[2] I almost stopped at this point, as this demonstrates well enough what I was out to show. I set the lip of the bottle close to the water surface to be reasonably sure that the surface water motion was representative of the amount of water going in and out.

[3] This takes a bit of practice. I can do it about half the time. I repeated it enough to be sure it wasn't just some kind of fluke.

[4] Substantially more water flowed out than would seem reasonable to expect being pushed in with my thumb as I close the opening, though this too would happen with an engine intake valve.

Can I get a bonus point on the test for this?
posted by sfenders at 7:46 PM on October 21, 2007

Hmm. That's intriguing but I'm inclined to think that the wave action you're seeing is the result of more than just filling the bottle.

But, you may be right.

In regard to the system in question, the difference between atmospheric pressure and a partial vacuum is not very large and a gas will move into equilibrium much faster than the water in your system. If you could time the valve to close just right in the combustion chamber, I doubt you'd get anything more than very slightly greater than atmospheric pressure, so there'd be no point to doing it. As it is, I bet that the compression cycle begins even before the chamber quite reaches STP.
posted by Ethereal Bligh at 10:15 PM on October 21, 2007

I doubt you'd get anything more than very slightly greater than atmospheric pressure, so there'd be no point to doing it.

Well yeah, I agree it's not of any real practical use here.

It's still typically less than atmospheric pressure, it's intake manifold pressure you'd compare it to, which in my car reads at 0.1 to 0.5psi below atmospheric pressure at full throttle, depending on engine speed. It's way lower (often more than 10psi difference) when I'm lighter on the gas pedal. Still, I suspect engine designers do try to close the valve at exactly the right instant for best performance. It's complicated by the fact they're also injecting pressurized fuel; I imagine they adjust it by trial and error, rather than trying to calculate everything. Now that I think of it, I have seen at least one manufacturer of after-market air intakes claim that they take into account resonance at certain engine speeds to smooth out some part of the torque curve. If they weren't just making that up to sound impressive, presumably they are adjusting it so that the pressure wave formed when the intake valve closes reflects back to the inlet just when air is needed.
posted by sfenders at 9:17 AM on October 23, 2007

Intakes have a huge effect on the intake velocity and air charge of NA engines. An extreme example is the cross ram intake on 60's Chrysler big blocks. Chrysler engineers developed special variable intakes with an action like a trombone to allow them rapidly test different intake lengths at different rpms. The goal is a good sonic resonance.
From the last link:

Someone, I think a brilliant engineer by the name of Bob Graham, deduced that if we tuned our intake runner to the point where the resonance was greatest, it would give the maximum push to the air and fuel when the intake valve opened at any given speed. The theory proved correct in the tests on the single cylinder and the results were reduced to a formula that was used from that day forward for ram manifolds on Chrysler engines. The runner as measured from the valve seat to the plenum (the open area where they normally meet under the carburetor,) can be determined by dividing 84,000 by the length of the runner = the speed the runner will work the best. An example is:

84000 (constant)/16 (runner length) = 5250 rpm

The formula worked with all camshaft designs tested, engine displacements, compression ratios, and bore and stroke combinations of the time.

Also int he vein of smoothing out torque curves:

Lots of cam and intake combinations were tried, but the best was the “Squid,” so named for the sea creature that it resembled. With its long runners it was tuned for 5500 rpm. We tuned runners for speed less than the point that the engine produced max power so as to give lots of power when rpm was down coming out of the turns at Daytona. Maximum power was at 6500 rpm.

The modern hemi is outfitted with computer controlled gates in the intake in order to allow the engine computer to vary the length of the intake depending on rpm.
posted by Mitheral at 9:47 AM on October 23, 2007

Nice link, Mitheral. This all has inspired me to do some more tinkering with my car.

From the wikipedia page on Helmholtz resonance:

When air is forced into a cavity, the pressure inside increases. Once the external force that forces the air into the cavity disappears, the higher-pressure air inside will flow out. However, this surge of air flowing out will tend to over-compensate, due to the inertia of the air in the neck, and the cavity will be left at a pressure slightly lower than the outside, causing air to be drawn back in. This process repeats with the magnitude of the pressure changes decreasing each time.
posted by sfenders at 4:22 AM on October 24, 2007

I stand corrected. I'm glad to learn something and correct my misunderstanding.

Still, it should be made clear to observers that these are marginal improvements relative to using a mechanism (like a supercharger or turbocharger) to truly compress the air before intake.
posted by Ethereal Bligh at 6:58 AM on October 24, 2007

Dang it. I just found this through another source and realized I missed a good fpp by about a week.
posted by Doohickie at 11:09 AM on October 26, 2007