TBR Physics Chapter 3, Passage 7, question 45

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BBlocker

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Hi guys,

Can someone please clear up this confusion for me?

45. Compare the work done by gravity on the block for each of the three ways of lifting the block.

A) I is largest
B) II is largest
C) III is largest
D) The work done by gravity is the same in all cases

Ans: D, The work done by gravity is the same in all cases.

The passage has a picture which would be difficult to reproduce here, but the scenario is that we have platform with height h onto which a box is being lifted (case I), platform with height h with an incline of 60 deg and platform with height h with an inclide of 30 deg. In each case we are lifitng a box onto a platform. We have to compare the work done by gravity on the block for each of three ways of lifitng the block.

So we have Wdone by lifter - Wdone by gravity = mgh. Correct? Yes, PEf = mgh, but to get to this point, work done by lifter and work done by gravity vary because of distance of incline, right? How can we assume right away that the work done by gravity is the same in all cases, don't we have to account for the work done by lifter (which varies, depending on the angle of inclination)?

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Hi guys,

Can someone please clear up this confusion for me?

45. Compare the work done by gravity on the block for each of the three ways of lifting the block.

A) I is largest
B) II is largest
C) III is largest
D) The work done by gravity is the same in all cases

Ans: D, The work done by gravity is the same in all cases.

The passage has a picture which would be difficult to reproduce here, but the scenario is that we have platform with height h onto which a box is being lifted (case I), platform with height h with an incline of 60 deg and platform with height h with an inclide of 30 deg. In each case we are lifitng a box onto a platform. We have to compare the work done by gravity on the block for each of three ways of lifitng the block.

So we have Wdone by lifter - Wdone by gravity = mgh. Correct? Yes, PEf = mgh, but to get to this point, work done by lifter and work done by gravity vary because of distance of incline, right? How can we assume right away that the work done by gravity is the same in all cases, don't we have to account for the work done by lifter (which varies, depending on the angle of inclination)?

Actually, BBlocker, the work done by gravity is the same in all cases, b/c PEf-PEi = mgh in all cases. Since PEf = mgh in all cases, the lifter must have at least done mgh joules of work. Since he is fighting against gravity, and he wants the final PE to be mgh, he must do 2mgh worth of joules of work to achieve the final PE =mgh. Oooh, I see, BBlocker, Thanks! You're welcome, anytime ;)
 
I see gravity is a conservative force ( so are magnetic and electric )
When gravity acts on something ( like making a something slide down a hill ) that something will lose exactly as much potential energy as the gravity force does work on the rock. Good stuff
 
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Remember that gravity is a conservative force. The path doesn't matter; the only thing that matters is the height h. So W=-change in PE, and since the change in PE is the same in all three scenarios (since gravity is path independent), W is the same. Hope that helped!
 
Remember that gravity is a conservative force. The path doesn't matter; the only thing that matters is the height h. So W=-change in PE, and since the change in PE is the same in all three scenarios (since gravity is path independent), W is the same. Hope that helped!

Yea, thanks. I somehow forgot that gravity is a conservative force.
 
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