# Mechanical Energy

1. Mar 1, 2009

### cybernerd

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

A 0.13 kg ball moves at a rate of 20.0 m/s at the point where it is 5.00 m above the ground. How much mechanical energy does the ball have with respect to the ground?

2. Relevant equations

Ek = 1/2mv^2

3. The attempt at a solution

Ek = 1/2mv^2
Ek = 1/2(0.13kg)(20m/s)^2
E = 26 J

The assignment says my answer should be 32.4 J.
Obviously, I am doing something wrong, but I'm not sure how to incorporate the "with respect to the ground" bit into my equations.

Somebody offer me some guidance, please?

2. Mar 1, 2009

### Sumanyu

I can't comment on WHY this is so.. but apparently.. the answer includes the potential energy of the ball as well

= mass x gravity x height

= .13 * 9.80 * 5m = 6.37J

26 J + 6.37J = 32.4J

Not really helping but I'm not familiar with mechanical energy...

3. Mar 1, 2009

### cybernerd

Hmm...that is strange....I really don't like the way that the question is worded.

But that did help, thank you.