Mechanical aptitude "test"

[Boundary Layer]

Right. Velocity, speed it's all the exact same thing to the layperson

heh, I'm thankful they chose not to butcher that topic on the test. It would've been another one I got wrong.


"If Edsel drives 30 miles in 30 minutes and then returns to his starting point by traveling back over the same 30 miles in another 30 minutes, what was his velocity?"

a) 0mph
b) 60mph
c) 30mph
d) Edsel broke down on the way back

My money says the creator of the test would've scripted b) into the answer key.

 

[backspace]

98%.

Got #7 wrong, which was the "match the drive ratio type" wrong. Screwed it up horribly.

 

[Azuremen]

Ok, ok now I see what they and you are saying. I thought all 3 were right next to one another, in the exact same environment and thus all sharing the same atmosphere. So in my mind the only reason for the volume differences was due to variable elasticity of the balloon material. I was reading it as a trick question obviously, thinking too deep into it.

I did the exact same thing. As engineers and such, its our job to be anal about details and such.

That drove me nuts... by the wording, the units should have been in lbf or N. If it were force the reaction would be equal and opposite, and as I recall, that wasn't even an option, so I made the assumption that the units were correct and that the creator of the test was too ignorant to distinguish force from pressure.

edit: that "which wrench is better" question is open to speculation as well... I mean, I know that I can't always fit the ideal wrench onto a nut/bolt due to space limitations. The long wrench might make the task significantly more difficult.

Oh, now that's just nitpicking.

And thus is the way of life for an engineer. A friend of mine finally figured out why I make some simple things so complex when I helped her with non-calculus based physics... in that I was reading 10 times more into the problems than there actually was.

heh, I'm thankful they chose not to butcher that topic on the test. It would've been another one I got wrong.


"If Edsel drives 30 miles in 30 minutes and then returns to his starting point by traveling back over the same 30 miles in another 30 minutes, what was his velocity?"

a) 0mph
b) 60mph
c) 30mph
d) Edsel broke down on the way back

My money says the creator of the test would've scripted b) into the answer key.

Yup, I would agree. Much like mass and weight are the same thing to the layperson.

Which is why we, the crazy people, get confused sometimes when lay people explain things. Anyone that isn't an engineer may notice we, engineers, tend to want very exact explanations on certain things. Its for reasons such as these.

 

[Alfaholic]

96%. I got question 39 wrong: what mechanical motion principle do automotive brakes apply. I said acceleration, correct answer is friction. I thought friction was a force instead of a principle.

I also got Q44 wrong. The decreasing diameter pipe with the two vertical pipes on it. My fluid dynamics let me down. I expected the decrease in diameter to accelerate the fluid and cause an increase in pressure so I said tube B would be higher. If that were true, then those venturis on F1 cars would make them take off instead of sit tighter on the road

I understand that the reason air enters is both outside pressure and suction, but I'm confused why the question made you pick one or the other.

Because the mechanism that moves air into the cylinder is pressure. The "suction" in this case is simply an apparent product of the pressure differential, much like "centrifugal force" is a apparent product of a centripetal force. Atmospheric pressure is the mechanism that fills the cylinder. It is more correct to say that the air is pushed into the cylinder by atmospheric pressure than it is to say that the air is sucked in by the vacuum created by the piston's motion, because air pressure is a force at work, while a vacuum applies no force to air molecules. A vacuum does not attract matter.

 

[Philly]

I also got Q44 wrong. The decreasing diameter pipe with the two vertical pipes on it. My fluid dynamics let me down. I expected the decrease in diameter to accelerate the fluid and cause an increase in pressure so I said tube B would be higher. If that were true, then those venturis on F1 cars would make them take off instead of sit tighter on the road

Care to explain which is right and why? I also thought that the water in the smaller tube would be faster and have more pressure, causing there to be more water in tube B.

______|A|________|B|________
______________________
__________/.................\_________

Just a crude drawing.....

 

[Metar]

So the more surface that is in contact with the surroundings, the faster it cools. Therefore, the smaller surface area will take longer to cool.

👍

Hence why Champagne is drunk out of tall, narrow glasses. To see the bubbles rise, and enjoy a cooler drink.

Care to explain which is right and why? I also thought that the water in the smaller tube would be faster and have more pressure, causing there to be more water in tube B.

______|A|________|B|________
______________________
__________/.................\_________

Just a crude drawing.....

Between A and B, the radius decreases, so pressure is building up there. As a principle, pressurized gas or liquids , and just-about-anything, will always go towards the point of the least pressure - in this case, Pipe A is the natural way to go for them. Pipe B is pretty unrelated in this case.

 

[Keef]

98%.

Got #7 wrong, which was the "match the drive ratio type" wrong. Screwed it up horribly.

I got that one right the first time without much thinking, but that's just because I have this nifty animation of a transmission at work implanted in my brain somehow.

👍

Hence why Champagne is drunk out of tall, narrow glasses. To see the bubbles rise, and enjoy a cooler drink.

No! You're thinking too much like an engineer. They use a tall glass because it concentrates the aroma, so when you whiff it the delicious smell is strong.

Between A and B, the radius decreases, so pressure is building up there. As a principle, pressurized gas or liquids , and just-about-anything, will always go towards the point of the least pressure - in this case, Pipe A is the natural way to go for them. Pipe B is pretty unrelated in this case.

One thing I was thinking about is how the water would flow under pipe B. Woud it flow smoothly? If you looked down into it would you just see a stream of water? Would the water hit the edge of pipe B and splash up, thus "filling" the pipe with its splashes? Would it rise and appear to not move at all?

So, how much would it rise in pipe B, generally speaking? Obviously less than A, but would it be close, or maybe just barely risen?

 

[Boundary Layer]

Care to explain which is right and why?

See post #33


edit:

Would it flow smoothly?

Odds are it would be turbulent flow, not laminar, but there is not enough information given to solve for Reynold's number to say for certain.

So, how much would it rise in pipe B, generally speaking? Obviously less than A, but would it be close, or maybe just barely risen?

There is simply not enough information given to even speculate a guess. It depends on the dimensions of the pipe prior to and at the restriction for determining area ratio, the surface roughness of the pipe, how abruptly the radius changes (ie. the included angle), the diameters of pipes A and B, atmospheric conditions, the static pressure of the flow (or the work or change in head produced by the pump and backsolve for pressure), the viscosity of the fluid, etc, etc...

Just stick with 'less than in A'

 

[Metar]

Hence why Champagne is drunk out of tall, narrow glasses. To see the bubbles rise, and enjoy a cooler drink.

No! You're thinking too much like an engineer. They use a tall glass because it concentrates the aroma, so when you whiff it the delicious smell is strong.

👍


As for the water-flow in the pipes, that really depends on the water-pressure. I'd guess it'd still rise quite a bit as long as the pressure isn't too small, or the diameter-difference between the wide section and narrow section isn't too big.

(My gut-feeling tells me at half the pipe-size, it'd rise half as much as the other one. But, that's just a gut-feeling)

 

[Alfaholic]

Care to explain which is right and why? I also thought that the water in the smaller tube would be faster and have more pressure, causing there to be more water in tube B.

______|A|________|B|________
______________________
__________/.................\_________

Just a crude drawing.....

Ummmmm I'd love to but I've just realised that my F1 car venturi analogy was incorrect.. that's more like a narrow pipe going to a wide pipe! So I'm not actually sure why A is the correct answer for the pipes. It does mean that the pressure is higher in the wider pipe, but I can't say why. Someone earlier mentioned something about the water speeding up under B, and the resultant reduction in pressure being due to that, and that sounds about right, but I can't actually picture the mechanics behind that.

EDIT: but, this might work:

It may be analogous to roadworks on a motorway, where three lanes have to filter into a single lane. In the three lanes, you have lots of cars in close proximity, all wanting to to travel down the single lane but getting eachother's way, so there is a high concentration of cars before the restriction - i.e. higher density upstream of the pipe reduction. Once you get onto the single lane, things spread out again. You go faster and spread out to a natural comfortable distance from the traffic around you, hence lower pressure downstream of the restriction.

So, in the pipe system from the question, you have a force causing the water to travel from the left to the right. The water obeys this force but meets the pipe restriction, so much of the force pushing the water through the pipe is used trying to press more water into the large pipe. More material in the same volume = higher pressure. This pressure pushes some water up pipe A and some into the reduced diameter pipe. Once the water reaches the reduced diameter pipe, it is free to go where it is pushed to. The restriction on its progress is reduced, so when you push at it, all the force goes into moving the water, none goes into attempting to compress it and raise its pressure, so its pressure reduces to meet a new, less stressed equilibrium. If the reduced diameter pipe is still completely full of water, then it will still be exerting some pressure on the pipe walls, and thus some water will be pushed up spout B, but not as much as A. I would guess that if you looked down into spout A this water would appear to be fairly calm, unless the pressure driving the whole system was fluctuating...

EDIT 2: Could it be explained by that gas equation... something like pressure = temperature x volume? Sorry... it's been 15 years since I studied fluid dynamics... Anyway, if that does apply, then you can perhaps treat the pipe before the restriction as a closed container of fixed volume. Add energy to the system, (like pushing against it) and the pressure increases. After the restriction, the container could be regarded as open, so the volume is not fixed. Add energy to that and the volume increases rather than the pressure.

Perhaps..

That's all guesswork, so don't quote me in your fluid dynamics exam...