Kinetic and gravitational potential energy

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Homework Help Overview

The discussion revolves around the relationship between kinetic energy and gravitational potential energy in the context of a skier descending a hill. The original poster seeks to understand why the kinetic energy at the bottom does not equal the gravitational potential energy at the top.

Discussion Character

  • Conceptual clarification, Assumption checking, Exploratory

Approaches and Questions Raised

  • Participants explore the idea that gravitational potential energy is relative and question the assumptions about where potential energy is defined as zero. They discuss the implications of energy loss, particularly in relation to thermal energy and friction.

Discussion Status

The discussion is active, with participants questioning assumptions and exploring different interpretations of energy conservation. Some guidance has been provided regarding the role of friction in energy loss, but no consensus has been reached on the overall explanation.

Contextual Notes

There is an ongoing examination of the definitions of potential energy and the reference points used to measure it. Participants are considering how energy transformations occur in the presence of friction.

pbonnie
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Homework Statement


Explain why the kinetic energy of the skier at the bottom of the hill is not equal to the gravitational potential energy of the skier at the top of the hill.


Homework Equations


n/a


The Attempt at a Solution


I think the answer is because the bottom of the hill is not at surface level, and therefore there is still gravitational potential energy. I just wanted to double check and make sure I'm not missing anything.
Thanks!
 
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pbonnie said:

Homework Statement


Explain why the kinetic energy of the skier at the bottom of the hill is not equal to the gravitational potential energy of the skier at the top of the hill.


Homework Equations


n/a


The Attempt at a Solution


I think the answer is because the bottom of the hill is not at surface level, and therefore there is still gravitational potential energy. I just wanted to double check and make sure I'm not missing anything.
Thanks!

I don't think that is the answer. If you define the GPE to be zero at the bottom of the hill and the skier starts from rest there, what else would keep the KE from being equal to all of the GPE when the skier reaches the bottom of the hill?
 
Potential energy is almost always a relative matter. I.e. it's the difference between two energy levels, rather than an absolute measure. (Sometimes the PE at infinity is considered zero, so the PE anywhere else is negative. This is commonly used in cosmological contexts.)
This makes the question a little unclear. You have interpreted it as taking zero PE to be at the centre of the Earth, right? Possibly, but I doubt that's what's intended. Why might the skier's KE not equal the difference in PE between top and bottom?
 
Oh okay. So does that mean that there is another kind of energy contributing to the total energy? So the kinetic energy is not equal because some of the energy was lost to thermal energy?
 
pbonnie said:
Oh okay. So does that mean that there is another kind of energy contributing to the total energy? So the kinetic energy is not equal because some of the energy was lost to thermal energy?

Yes. What caused that loss?
 
Friction?
 
pbonnie said:
Friction?

Bingo!
 
Great, thank you both very much!
 

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