Physics of Engine Repair

Don't engineers and technicians generally use pounds and kilograms (as in, 9.807 N) to measure forces? I think physicists and some more finicky engineers just call them "pound-force" and "kilogram-force".

Since when is two equal weights hanging on either side of a pully considered a stable configuration?

86% (pass). Think I mostly messed up on the pully questions.

Is there any way to get feedback (esp. on what I got wrong, and why the correct answer is correct)?

You should warn your students it's large (slow to load) and the font is really tiny (questions are hard to read).

such.ire:

No. At least, we read parallel basic courses, and newton was the norm. But in some professions they may have their own conventions.

Aaron Bergman:

Marginally stable?

Misuse of units is a bad practice that should be eradicated and never be tolerated.

My first visit to a deep space station, I saw a panel labeled '10 MC' and my blood ran cold. A single coulomb is a lot of charge, but ten million? Then I found out that was sloppy engineering usage and meant 10 MHz.

The ratio of signal power to noise density has dimensionality of reciprocal time, so the units Hz and dB-Hz are appropriate, but engineers in the space biz often get sloppy and give no units for the linear value, and dB for the logarithmic value.

In radio communication, carrier suppresssion can be expressed different ways, but the value is a fraction in the interval [0,1) and so the decibel expression of a the fraction is negative. However, many engineers will write '3 dB' (or worse, '3 DB', or '3 db') rather than -3 dB.

My practice, whenever I encounter 'kg (wt)' or 'lb-m', is to redline it and send it back for correction.

Is there any way to get feedback (esp. on what I got wrong, and why the correct answer is correct)?

There's a link that looks like two rectangles in the lower-left at the end.

90%. I bet I missed the same ones you did.

Since when is two equal weights hanging on either side of a pully considered a stable configuration?

Well, it is an equilibrium situation (no net force), so if the masses aren't moving initially, they'll continue not moving. It's not a stable equilibrium, though, because if you start them moving in one direction, they'll keep moving in that direction at constant speed (ignoring friction and air resistance). It's not unstable equilibrium either, though, because it doesn't accelerate away from the equilibrium position after a small displacement. I think the intro mechanics book we use calls this "neutral equilibirum."

It's stable compared to the other case they gave, but I agree that it's a slight misuse of the term, from a physics perspective.

I messed up the pulley ones too (took that stuff a while ago). Here's the secret to remember.

To lift the mass a certain distance will always require the same amount of work. Work = Force * Distance. So, if you increase the distance that you're pulling across (e.g. by using a bunch of pulleys), then you have to decrease the force by the same factor.

A thing I like of engine-ers when confronted with the unit system is how elegantly they retort it. They have two units, the Joule and the NewtonMeter, to differentiate if they are speaking of the scalar product or the cross product.

I got 44/50, only one point less than a physicist!!! Yesssss!

Well I got "470 points or 94%,", so I must have missed 3. I probably spend more time around diesel engines than most as I know how to fix a forklift. And my nickname at the job site alternates between "Mr. Leverage", "Mr. Science", and "Houdini".

The bad units on question 41 had me confused as the appropriate unit for force is pounds, not psi. And there were a few questions where one has to think like a diesel mechanic.

As a youth, I considered joining the military. They had an aptitude test that was similar to this, but with 25 questions. They gave you some unimaginably long period of time to work it. The only question I had to guess about was one where they gave you a picture of one of those battery testers that measures specific density, and asked whether the battery was charged or discharged. Since I'd never used one, I had to work through the density changes to the working fluid in the battery, which is to say I had to guess.

I got 100% - despite knowing that force isn't measured in kg or psi. I'm certainly no diesel technician, but I have at least changed the oil in my car. Maybe hanging around with a heavy-duty mechanic who works on big diesel engines has given me a bit of an edge? I agree that some of the questions were poorly worded, from a physics perspective. The "stable"/neutral equilibrium pulley question is a good example of that.

re: checking answers - it is the magnifying glass icon at the end to review.

re: units - yes, very sloppy, particularly the psi. I don't think that can be excused. Re: the kg - these guys are used to calibrating force-measuring devices with the force a mass exerts. Sloppy, but exactness doesn't help them. Torque as force is sloppy, too, and more serious.

re: orientation - the beveled gears involve a particular orientation assumption. That assumption does not work on the fan question. I admit, there is support for their position on the fan question, but there is support the other way, too. (Yeah, I got it wrong :<( - and I was sloppy on the multi-pulley single belt one.)

re: cooling - well, it was nice of them to draw the wavy lines.

After all the sloppiness, they got pedantic about suction vs. air pressure - at least they did it correctly. That distinction is more important than pressure vs. force for people working on hydraulic systems? Well, they do have to distinguish various air feed schemes.

If you give it to your students, they may learn more from the test's drawbacks than from what it is trying to test. ;<)

92%! Not bad, considering it's been seventeen years since my last physics class ...

To hell with lbf, this country needs to get with the rest of the world and start using SI! I took that test, and part way through, it would'nt let me give an answer so I just exed out of it. I had a bad teacher for Design of Mechanical Components, so I don't know much about bevel gears, etc. I can only solve those problems intuitively (especially since therer were no measurements given on the other gears.)

Hmm, actually, what is the US equivalent of the Germanish "NewtonMeter" I mentioned above?

I may make my students take it the next time I teach intro mechanics...

I'm reminded of my pal who used to claim that he took his VW to a Quantum Mechanic.

88%, got a couple wrong just to rushing..

Hmmm, am I qualified to repair the hi-tech computer-controlled HDi engine crammed into the engine bay of my Pug now?

98% I wish I knew about the magnifying glass thing so I could see which one I missed.

I didn't really think about the air is sucked in/pushed in question at the time, but Richard is right. I hate when people get all smug about some nitpicked detail that isn't really even wrong, and then screw up the most basic concepts.

I wasn't so irked by the kg/lb in this test, because your average diesel mechanic (apparently, I could be one 84%!)was taught science and SI by our average school teacher who doesn't make the distinction between mass and weight (I did.). I hate when I see mass and weight used interchangeably in science books! If science books (usually not text books) can't get it right than how will we ever teach the public to get it right?

A friend of mine has a book that explains that on the moon you would weigh 1/6 as much because the force of gravity is the 1/6 as strong. For example if you weigh 120lbs on Earth, you weigh 20lbs on the Moon, OR if you weigh 60kg on Earth you weigh 10kg on the Moon. Almost makes me pull out my hair (her, too).

I think the intro mechanics book we use calls this "neutral equilibirum."

Thanks, sounds familiar, and decidedly more correct both ways than "marginally" and "stable".

98%! And I'm an ignorant electronics technician who has never seen the inside of a college classroom, and the sum total of my automotive knowledge is how to find a good mechanic.

I wish I had noticed the link to find out which question I missed. It was probably either the one with the two tubes and the tapered pipe (I've never had a good "feel" for hydraulics or pneumatics) or the one with the fans.

The fan question is ambiguous- "same direction" and "opposite direction" aren't defined as being seen from the same point of view or as seen from the fan end of each shaft...