# Homework Help: Potential Energy and Systems

1. May 22, 2013

### whitehorsey

1. When taking an object and the Earth as a system, its change in potential energy is given by
a. m*g*change in height
b. (-m)g*change in height
c. either a or b
d. there is not potential energy due to gravity

3. I think it is c because the sign of energy does not matter?

2. May 22, 2013

### ehild

The sign does count. Is the potential energy of the Earth-object system negative or positive if you take the zero of potential energy at infinite separation?

ehild

3. May 22, 2013

### whitehorsey

What's infinite separation?

4. May 22, 2013

### ehild

I meant infinite distance between the centre of Earth and the object where the potential energy of the whole system is zero. The force between them is GmM(earth)/r2, towards each other. That corresponds to the potential energy of interaction U=-GmM/r. Does the potential energy increase or decrease if the object gets farther from the Earth centre?

ehild

5. May 22, 2013

### whitehorsey

Oh! Decrease?

6. May 22, 2013

### ehild

Oh not! The magnitude of a negative value decreases, what does it mean???

ehild

7. May 22, 2013

### whitehorsey

Ah it becomes positive! So a?

8. May 22, 2013

### ehild

more positive.

Near the Earth surface, the force on the object is mg (downward). Does the potential energy increase or decrease if the object raises by h?

ehild

9. May 22, 2013

### whitehorsey

PE = (-mg)*(+h) = -mg. So PE decreases too which leads to a?

Last edited: May 22, 2013
10. May 22, 2013

### ehild

No, the potential energy is the work against the force of gravity... The work needed to lift the object...

ehild

11. May 22, 2013

### CAF123

You have found the work done by the force of gravity as the object is moved a height h. How are potential energy and work related?