# Homework Help: Change in PE Question

1. Sep 26, 2009

### jpartdq

1. The problem statement, all variables and given/known data
. Consider the energy of a 100 kg rock.
a) What is the gravitational potential energy (PE) of the rock that is 5 m above the ground?

b) The rock falls to the ground. What is its PE?

c) What is the change in PE of the rock?

d) What is the change in the kinetic energy of the rock?

2. Relevant equations

MGH

3. The attempt at a solution

a) 100*9.8 m/s2*5 = 4900 J
b) 0J
c) Change in PE = 4900 J
d) Change in KE = ½ * 100kg * (9.9 m/s)2 = 4900 J

Can anyone look at my answers?

2. Sep 26, 2009

### Fightfish

Looks correct assuming that the surface of the earth is taken as the zero reference point. Should include +/- signs for the "change in XX" to indicate whether its an increase or a decrease. What's with the strange working for change in KE though? Since no external forces act on the system, the change in KE is simply equal to the change in PE magnitude-wise.