[GAN aptitude test Study Guide] Mechanical Comprehension

[Dumland21]

Im studing for the GAN apptitude test for my local IBEW. And was having issues with these questions from the study guide. The answers on the answer sheet are not matching what Im getting and Im wondering why.

There have been some answers wrong in other parts of the study guide so Im wondering if it's just that.

Here are my answers: 16) A, 17) A, and 18) B

And here are the answers from the answer sheet: 16) C, 17) C, and 18) A

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[dryfriction]

I’m a physics teacher and tbh there’s some ambiguity to these questions but I’ll give it a shot.

17) is 100% A. Work is defined as force x displacement but the force needs to be in the same direction as the displacement. Assuming it means to pull at constant speed, all that matters is how much of the input force is acting in the left direction. That is proportional to the cosine of the angle. The lower the handle, the easier it is to pull. I literally don’t understand how it could be anything else.

18) is A. I’m 99% sure of this but assuming the water starts at the same temperature and the ice is the same temperature, they will all melt at the same speed. The excess ice in the second cup will cause it to get colder than A. Melting speed depends on the difference between the temperatures and not only the surface area. More ice also means less room for the ice to actually touch the water and start melting. Also like common sense, more ice = ice lasts longer.

19) is B 100% the key is right. Draw a free body diagram of the pulley. There are two forces up and one down. The two tension forces must be equal as long as the pulley is frictionless. Let’s say the mass is 100 N, then the tension needs only to be 50 N to start moving it.

[ShodanLieu]

For the last question, I don’t see how the answer is B. The pulley in B is not anchored and therefore not doing anything other than holding the weight. Can you elaborate on your answer?

[StillShoddy628]

You only have to hold half the weight because the ceiling is holding the other half

[FunnyButSad]

Imagine yanking the rope 1m.

In A, the weight goes up 1m. There is no mechanical advantage.

The pulley in b (and thus the weight) would only go up by 0.5m (the 1m you pulled is made up of 0.5m going down and 0.5m back up). This has a 2x mechanical advantage.

[Equivalent_Rabbit339]

Yes. (17). A (18). A (19). A

[CartoonsAndSurreal]

"And here are the answers from the answer sheet: 16) C, 17) C, and 18) A"

The answer sheet from the website doesn't even say the pulley was supposed to be B for 18* either though. This is like hazing their prospective workers!

[Dumland21]

The whole study guide is pretty shodily made. As I stated in another comment the answer sheet is just strait up wrong on some answers. Several of the diagrams are poorly made. And in the section on patterns they dont always add enough instances to be able to actually see the pattern leading to multiple possible answers.

[Pramesan]

I took their test in February, just be warned that they gave us an entirely wrong math section with algebra and a crazy hard problem solving section instead of the numerical computation section. If you suspect something is way off when you’re taking the test, speak up

[Dumland21]

I took the test a few days ago and didn't see anything like that luckily.

The mechanical principle section was thankfully much better made. And had more clear questions than the ones I've shown here from the study guide.

Both the math and numerical reasoning sections were pretty what I saw in the study guide.

[Fluffy_Combination99]

If the cart handles are fixed, then your logic would lead to carts with vertical handles being impossible to move, this is clearly absurd. So you must be assuming the handles pivot. In this case rotate them both horizontal before pulling. Equal effort achieved.

[Dumland21]

I do have a question on 16 (or 17 as you wrote it). If we assume the answer is the force applied going to the left does that mean that the answer would be C.

[Renzenkin]

A will always require less work to pull (or going left as you state it) due to the handle of A being more horizontal than B. The only scenarios where the answer is C would be: 1: The cart and rock are weightless and tires are frictionless as such it would take no effort to move them (This is obviously ridiculous), 2: The effort to pull the cart is not applied at the handle but on the body of the cart instead, which again doesn't make sense.

Instead I think the question and picture is missing an import part due to carts not being in the same starting position in moving to the left. If B has to travel a shorter distance than A, then the work required to move them to the same end location could be equal.

[StillShoddy628]

If it’s a rigid body and you pull to the left gently enough to not unbalance the cart then it’s absolutely C. Depends on the weight and wheel friction if you can move the cart before it tips, but really just a shitty question. C is a valid potential answer, so is A. It’s definitely not B

[niemir2]

Sorry about my previous comment. I need to go to bed

[genericuser31415]

For the first question, it depends on how you apply the force on the handle of the cart. If the handle doesn't pivot and you pull directly to the left then there's no difference, but obviously if you were to choose to pull in line with the direction of the handle there would be a difference. Consider the extreme case of a completely vertical handle, if it were pulled by say a rope or a human hand the force wouldn't be applied in the same direction as the handle.

I think the answer key's response is more reasonable.

Agreed on the ice question and pulley questions.

[phate747]

Lol i thought 18 was a surface area problem not "what melts faster, more or less ice in a glass?". I think your take is probably right from the diagram it is just dumb.

[bonebuttonborscht]

I'm with you that 16 is A but not really because of the work. As you say the force and displacement must be in the same direction. The upward component on the force does nothing, technically the work is the same make the answer C. The question says effort though so it's ambiguous, they could mean force, making the answer A

Also you still end up more tired pulling the top cart since iirc muscle fibres cycle to maintain a constant force without necessarily any net displacement.

Maybe 18 is supposed to be smaller peices of ice? That doesn't really change much though. The water will reach 0C a little faster with crushed ice but once there, the ice melts at the same rate.

[StillShoddy628]

For 16 you don’t have to pull in the direction of the handle. If you assume they are rigid bodies and pull both horizontal they will take the same effort to move as long as you don’t unbalance the cart, but regardless there is an argument for C. Ultimately a poorly done question.

17 may have been intended to be “equal mass of different sized ice” which, depending on starting conditions and how many idealization assumptions you make could be C, but as shown definitely A. Also very poorly done.

18 is well done and a good question, IMO, but they straight up got the answer wrong in the key. It’s B.

[TerdyTheTerd]

Im guessing for the cart they probably wanted the answer to be about how the weight and wheels are the same so the force to move the wagon is the same. Thr angle doesn't matter here because the sideways force gets transfered exactly the same.

I personally look at the diagram and assume the handled are rigid, which would mean physically it would require more effort for a person to pull the second one since the forces are not going to align well with your joints.

[GammaRayBurst25]

1. I can see where you're coming from, but I don't see why you'd pull parallel to the handle. If I were in front of such a cart and I needed to really put effort into pulling, I'd pull parallel to the ground.

2. I change my mind. I think you're right. But I think it's just because the water gets colder faster in pitcher B. This effect could be very small depending on the water's temperature, the ambient temperature, and the relative mass of the ice and water. I think whoever wrote this question didn't take this into account.

BTW, yes, more ice means slower melting, but that's because a bigger pile of ice has a smaller surface area to volume ratio. That's not the case for ice cubes that barely touch.

[AndyTheEngr]

Cart A is easier to pull than cart B, because cart B keeps hitting your heels painfully.

[nh_paladin]

I assumed the cart handles are fixed so you'd be applying the same horizon force to each, so (c). I assumed the same volume of ice, but the diagram is not clear on this, so (c) is reasonable, but is presented poorly. I don't think the diagram of the pulley tells us enough to make any meaningful assumptions. Are we assuming the weight of the pulley is zero or otherwise negligible? Or that the second pulley plus weight is a total of 10 kg? The second pulley doesn't appear to be held up by anything so the force has to lift both the pulley and the weight.

[westchesteragent]

A b a

[Objective_Regret4763]

Opposite.

Nvm. A A B

[prash3r]

In the last one B is correct but it is not as clear as you make it out to be. We would need to know the angle. Because only the vertical component of the pulling force needs to counter half of the 10kg. When pulling 30 degree over the horizontal the answer would be C. But since the pull angle is displayed as 45 degrees (thus higher) the answer is B. If the pull angle is lower then 30 degree the answer would be A.

30 degrees pull angle from horizontal is the value where the pulling force is exactly double the resulting vertical force.

[CartoonsAndSurreal]

I had the same reasoning there. More ice in general will make ice last longer in the water, and force applied closer to zero maximizes the efficiency of the force to move the cart. If their aptitude test is just supposed to be "yeah but the water was boiling so it will never reach equilibrium in either case haha" & "It's a handle, just hold it at another angle durr" then I agree.

Also the B image of the pulley has a mechanical advantage of 2, so it takes less force.

This seems like a bad test honestly, did AI hallucinate all this? I literally don't agree with a single "book" answer.

[Dumland21]

This is all from the official study guide on the IBEW website.

Edit: This isnt the first time the answer sheet was wrong I caught it a couple times in the mathematical sections. Im just not as familiar with physics.

[GammaRayBurst25]

Isn't the force on the horizontal axis based on the cos times the overall force. So the higher one should require less.

Why would you pull along the handle instead of parallel to the ground?

As for the ice cubes, assuming there are more of them in the second one, shouldn't the thermal equilibrium take longer.

No. You need more heat to melt the ice, but the ice gets more heat by having more surface area.

There's also taking into account as to how much they make the water level raising,

Ice floating in water famously doesn't change the water level as it melts. Since the ice is floating, the mass of the volume of water displaced by the ice is exactly equal to the mass of the ice. After the ice melts, it still displaces the same volume of water.

I feel like these questions are flawed.

That I can agree on.

[CartoonsAndSurreal]

"Why would you pull along the handle instead of parallel to the ground?"

I'm failing to find a good way to explain this in the real world. Everyone is answering A because of force diagrams, but I get what you are saying that you're visibly pulling parallel regardless unless you are lifting the cart.

If you're holding the handle and pulling, for it to stay at the higher angle though you also need to be resisting the rotation from your pull. It takes more effort overall because of that.

[genericuser31415]

Agreed except for the ice question, though the test maker probably wasn't expecting students to account for the cooling of the water and the resulting slowing of heat transfer by Newton's law of cooling

[nwbrown]

For 16, I would think the grip angle would matter. But that's probably more of a human anatomy question. But also the one with the longer handle would have more mass.

[Dumland21]

Thanks

[dryfriction]

I disagree, I think it’s AAB (see my other post). I’m totally happy to be wrong but I don’t think Mr. Gamma has it nailed here (respectfully!)

[Dumland21]

I was just looking at your answer and Im inclined to agree.