Potential Energy & Niagara Falls

[warrior2014]

Homework Statement

Calculate the change in potential energy of 2.8kg of water as it passes over Niagara falls (a distance of 50m)

Homework Equations

U= mgh

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!

 

[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.

 

[NascentOxygen]

Would it be a negative number?

 

[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.

 

[NascentOxygen]

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

 

[warrior2014]

lose?

 

[NascentOxygen]

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

 

[warrior2014]

oh okay. thank you! now I understand.