Berkely Review Work Question

[Mces97]

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On page 129 in the physics books there is a question and it says we need to use W=PEfinal- PEinitial to get the answer, but I am a little confused. This is the question.
A 2kg cart starts on a 2-m high table before it slides down a 30 degree ramp with μk=.20 and then comes to rest at 1.31m from the ramp on a horizontal surface. How much work was done to stop the cart. I don't think we can just use mgh for the PEinitial and 0 for PEfinal because of the μk=.20 in the problem. Do I just use work = FxD, substituting 2x10x1.31, or do I have to use 2x10x1.31x.20. It does not say if the horizontal surface has the same μk. Thanks

 

[milski]

Actually, you can use ΔPE in this case. ΔPE should have been converted to KE and that was all the KE that object had. All the work does was to stop the object, which means that the KE=work done. So you kave ΔPE=KE=work done. You don't need to know μ - that would only change where/how soon the object will stop, not the amount of work done on it.

 

[Mces97]

So your saying that the answer is mgh, which is 2x10x2=40

 

[milski]

So your saying that the answer is mgh, which is 2x10x2=40

Yes.

 

[silverice]

thanks for clearing it up!!!!

 

[MrNeuro]

Yes.

correct me if im wrong here but isnt the change in potential energy mg2 - mg1.31?? which is 13.8?

Also another question when and how do use the change in potential energy equation to calculate work done?

 

[milski]

correct me if im wrong here but isnt the change in potential energy mg2 - mg1.31?? which is 13.8?

Also another question when and how do use the change in potential energy equation to calculate work done?

h is height, the horizontal displacement does not matter. The potential energy difference is m*g*2-m*g*0=2*m*g=2*2*10=40 J

Generally, it's best to use it when no other work was done on the object. Letting something fall down/slide down just from its weight is a good example.

 

[MrNeuro]

h is height, the horizontal displacement does not matter. The potential energy difference is m*g*2-m*g*0=2*m*g=2*2*10=40 J

Generally, it's best to use it when no other work was done on the object. Letting something fall down/slide down just from its weight is a good example.

oh i thought it was saying it slid down a ramp and stopped at 1.31 m in the vertical axis

as in the table top is 2 m (y-axis) slides down ramp to and stops at 1.31 m (y-axis)

 

[milski]

oh i thought it was saying it slid down a ramp and stopped at 1.31 m in the vertical axis

as in the table top is 2 m (y-axis) slides down ramp to and stops at 1.31 m (y-axis)

Yes, 1.31 m away from the ramp, wherever that happens to be. The cool thing is that you don't have to use that info to get answer. 😉

 

[MrNeuro]

Yes, 1.31 m away from the ramp, wherever that happens to be. The cool thing is that you don't have to use that info to get answer. 😉

lol thanks