# Homework Help: Potential Energy of a System

1. Jan 1, 2012

### Alaa Mahmoud

1. The problem statement, all variables and given/known data

a trophy being shown off by a careless athlete slips from the athlete's hands and drops on his toe.Choosing floor level as the y=0 point of your coordinate system,estimate the change in gravitational potential energy of the trophy-Earth system as the trophy falls.Repeat the calculation,using the top of the athlete's head as the origin of the coordinate.
The trophy has a mass of 2 kg,and the top of a person's toe is about 0.03m above the floor.Also the trophy falls from a height of 0.5m.

2. Relevant equations

(? U)=(U final)-(U inetial)

3. The attempt at a solution
I'm not sure how to get started as I'm not good in solving equations of energ.
I don't know how can I differentiate between potential energy & kinetic energy ??

2. Jan 2, 2012

### cupid.callin

Do you know any eqn of potential energy of a body wrt earth ?

3. Jan 2, 2012

### Alaa Mahmoud

Potential energy of gravity=mgy

4. Jan 2, 2012

### cupid.callin

Yes. So What would be PE of system when trophy is in athlete's hands?

5. Jan 2, 2012

### Alaa Mahmoud

PE=mg(y initial)=2*9.8*0.5=9.8J right??

6. Jan 2, 2012

### cupid.callin

Yes.
And similarly you can find the final PE ...
Then change will be ____ ?

7. Jan 2, 2012

### Alaa Mahmoud

OK thank you
the change in PE will be approximately 9 J
But is there any PE except gravitational PE???

8. Jan 2, 2012

### cupid.callin

Yes. There are electric, magnetic etc

9. Jan 2, 2012

### Alaa Mahmoud

OK thanks alot :)