TBR Physics Energy and Work Question #17

[jw1985]

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Hi All,

I don't quite understand why KEmax<PEfinal+KEfinal.

Shouldn't PEfinal = 0? Furthermore, if KEmax<PEmax, then isn't the loss of E through friction already accounted for in the value of KEmax. If friction is already accounted for in the value of KEmax [like what the answer says] then why should the values of KEfinal and KEmax be different?

The original question is copied below:

As a mining cart travels down the rails of a mine, what is true of its relative energies?


A.PEmaximum = KEmaximum
B. PEjnitiai = PEt + KEt
C. KEmaximum > PEfinal + KEfinal
D. TEmaximum > KEmaximum

The correct answer is D.

TBR's explanation for eliminating choice C:
At the bottom of the ramp, the cart has come to a point of minimal potentialenergy,and the cart has reached its maximum speed.This means that the final speed is the cart's maximum speed, which means that its final kinetic energy is the cart's maximum kinetic energy. The maximum kinetic energy (which occurs when the cart reaches its highest speed) is less than the sum of the potential energy and kinetic energy of the cart at the bottom of the hill(KEmax<PEfinal+KEfinal)- This eliminates choice C.

Thanks for your help in advance.

 

[onedirection]

I mean

The total maximum energy would kind of have to be greater than or equal to the Maximum kinetic energy by default right?

Total Energy = PE + KE



Even if PEfinal was 0, which is a good conclusion in a lot of questions, it would mean that KE is at it's maximum, therefore KEmax couldn't be greater than KEfinal, they'd be equal