# Potential Energy & Niagara Falls

1. Oct 12, 2011

### warrior2014

1. The problem statement, all variables and given/known data
Calculate the change in potential energy of 2.8kg of water as it passes over Niagara falls (a distance of 50m)

2. Relevant equations
U= mgh

3. The attempt at a solution

I plugged in the values into the equation U=mgh= (2.8)(9.8)(50)=1372J but it was the wrong answer. I'm not exactly sure what I am doing wrong. Any help is appreciated!

2. Oct 12, 2011

### PhanthomJay

You are calculating the PE at the top. What's the PE at the bottom? The problem is asking for the change in PE as the water falls from the top to the bottom.

3. Oct 12, 2011

### Staff: Mentor

Would it be a negative number?

4. Oct 12, 2011

### warrior2014

if the height at the bottom is 0, then wouldn't the PE at the bottom also be 0? so then PE would be 1372J? I'm kind of confused.

5. Oct 12, 2011

### Staff: Mentor

Does a body gain or lose PE as it falls to ground?

6. Oct 12, 2011

### warrior2014

lose?

7. Oct 12, 2011

### Staff: Mentor

So, in falling it loses PE. Hence the negative sign.

8. Oct 12, 2011

### warrior2014

oh okay. thank you! now I understand.