Random Questions

[ehara1]

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I don't know which thread to ask this on, so I figured maybe making a random question thread for people who just have random/general questions could ask them on here.

My random question is for anyone who has experienced the OAT, did they ever use units of pounds (lbs) on the test for Physics? On my Kaplan practice exams, they use units of pounds for weight when they're asking questions about a persons mass on an elevator, and it doesn't really seem like a normal thing because its much for difficult to work with pounds when you're trying to get an answer in Newtons.

If its against the rules of the OAT for anyone to answer this since it is talking about the test, then just ignore it cause I know people are not supposed to talk about the exam, but if it is ok since its not entirely specific to the actual material of the exam, if you could answer that'd be cool.

Thanks.

 

[Salmon]

No one can say for sure what will be on your OAT, everybody's experience is a little different. What I can say is that they probably wouldn't expect you to know the conversion of Pounds to Newtons (which is like 4.5N = 1 lb since it's a unit of force). Sometimes they'll mislead you and you may not even need to know what the units or values are to answer the question.

What was the question asking exactly? Could you write the question and list the answers? It might be easier to assess what your problem is.

 

[ehara1]

yea sorry. here's the question. maybe theres just an aspect to it I'm not understanding:

A passenger stands on a scale which is inside an elevator. If the scale reads 140 pounds when the elevator is at rest, what would it read when the elevator begins to accelerate upwards at 3 ft/sec^2? (g = 32 ft/sec^2)

I guess first off, I'm not used to seeing/using pounds, because when its newtons you can just use F=ma. Also, I've never used g = 32 ft/sec^2, or why that is used here

 

[EyEnStein 07]

they probably wouldn't expect you to know the conversion of Pounds to Newtons (which is like 4.5N = 1 lb since it's a unit of force).

Agreed, totally unnecessary to remember something like that. See below why.

yea sorry. here's the question. maybe theres just an aspect to it I'm not understanding:

A passenger stands on a scale which is inside an elevator. If the scale reads 140 pounds when the elevator is at rest, what would it read when the elevator begins to accelerate upwards at 3 ft/sec^2? (g = 32 ft/sec^2)

I guess first off, I'm not used to seeing/using pounds, because when its newtons you can just use F=ma. Also, I've never used g = 32 ft/sec^2, or why that is used here

So you dont really need to worry about "Newton" in this case since it gives you pounds. The answer choices will surely not be in newtons. They are giving you FPS units instead of MKS. You should be comfortable with both and commit 32 ft/s^2 to memory just as you have 10 m/s^2. Do you have the answer choices associated with this problem? The units must be in pounds for the answer choices. If not (which i highly doubt), it is worth knowing that 1kg = 2.2 lb

You should just proceed as you would normally. Remember that during the OAT you need to work through things as fast as you can. Since the elevator is accelerating up, you know that the scale would read something heavier than 140lb (therefore, if you see 4 answer choices below 140 and one above, you should pick that and be done with this problem). Why is that?

The Sum of the forces = ma

What are the forces acting on the passenger? His weight is pointed straight down. He has a normal force pointing up. The overall acceleration is the elevator moving up, ma.

What causes the normal force though? Its the scale that he is standing on. THATs what will tell you how much the scale reads. Important to remember.

So using sum of forces = ma:

I want to pick my axis such that everything that points up is + and everything that points down is -. Do note that if you chose to pick everything thats up as - and everything thats down as +, you will get the EXACT same answer.

+Fn (pointed up) - mg (the mans weight) = +ma (overall acceleration of the elevator going up)

=> Fn = ma + mg
=> Fn = m(a + g)

Look at that equation...its clear why moving up gives a larger reading on the scale.
So basically look at the units, it gives you a "force" that has the units of lb*ft/s^2 right? But we dont want the force, the problem asks us for what the weight is. Without even caring about a Newton or what a lb*ft/s^2...we can simply take the force and divide it out by g which is 32 ft/s^2.

So again, Fn = 140lb (35 ft/s^2) = 140(35) lb ft/s^2

F = ma --> m = F/a ; (we want the weight in pounds)
Now just divide 140(35) by 32 ft/s^2 and you will have your answer in pounds only (write out the units to see how they cancel).