# Homework Help: Kinetic engery

1. Oct 4, 2006

### rkslperez04

Im wondering how do we know when to substract Kinetic engery from potential engery or when do we add them?

Is that making sense?

question is asking:
a women skis downhill at a constant speed of 8.0 m/s when she reaches an icy patch on which her skis move with negligible friction. If the icy patch is 10 m high, what is the skier's speed at its bottom?

the book says to use this formula:
KE2=KE1 + PE

why are adding kinetic and potential together? I understand how to manipulate the equaiton to solve for speed but why are we using this formula?

2. Oct 4, 2006

### neutrino

The sum forms the total (mechanical) energy. In the absense of friction, it's a contant (conservation of energy). Since it doesn't matter where the "zero" of potential energy is, assume the skier has P.E = 0 at the bottom of the icy patch.

3. Oct 4, 2006

### rkslperez04

still lost...

When does it matter about the ZERO potential engery?

4. Oct 4, 2006

### neutrino

Did you read the page I linked to?

5. Oct 4, 2006

### rkslperez04

DUhhh... sure didnt.. I thought that was an underline.. BRB :)

6. Oct 4, 2006

### arildno

ALWAYS START OUT WITH CONSTANT MECHANICAL ENERGY.
On a given side, you will have the potential&kinetic energy for the same instant summed together.

7. Oct 4, 2006

### rkslperez04

ahhhh.... its amazing what happens with the light bulb comes on!

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