# Homework Help: Gravity potential energy

1. Apr 15, 2010

### Windseaker

1.4000lb of water are at the top of a 200ft. waterfall ready to fall. What is the amount of gravitational potential energy?

2. PE=mg(change in height)

3. PE= (4000lb)(1slug/32lb)(200ft)
PE= 25,000lb*ft
I think somethings missing

2. Apr 15, 2010

### collinsmark

Hello Windseaker,

I've always hated working any system that has units of slugs. http://www.websmileys.com/sm/aliens/hae48.gif

No seriously though,

You might want to look again at your question and equation carefully. Are pounds units of mass or of force? (hint: the units of PE in this system should be a dead giveaway on what went wrong.)

3. Apr 16, 2010

### Windseaker

Im haveing the basic problem of understanding the use of gravity in Lb./ft2 or slugs "itself" here. I thought it was a basic problem to understand it with. Is there any American unit help here?

(4000lb.=m)/(32 ft/s2=g)= 125slugs ?? 1slug= lb.sec2/ft

is this right?

4. Apr 16, 2010

### collinsmark

Yes, I believe you're right! (except for the '=m' part)

But let me give you a hint that might make the problem easier. In this problem, you don't need to bring slugs into it.

You've already stated that PE = mgh. We know from Newton's second law that F = ma. Gravitationally speaking, this becomes F = mg. Therefore gravitational PE = Fh.

And by the way, in case you didn't know,
Slugs are units of mass.
Pounds are units of force.

5. Apr 16, 2010

### Windseaker

PE = Fh
PE = (4000lb)(200ft)
=800,000lb(ft) ?

what happened to g

I was hopping to see how everything cancels out!

sorry , I just missed this, is this right?

if so can you help me understand Kinetic Energy of this problem (then water hits) as well??

Last edited: Apr 17, 2010
6. Apr 17, 2010

### Windseaker

OK, I believe this is right. except---ft*lbs

PE = mgh, F = ma, F = mg, Therefore gravitational PE = Fh

PE = Fh

PE = (4000lb)(200ft)
=800,000lb(ft) ?---------------800,000ft*lbs.

ft*lbs is a English unit for Energy and does not need to be converted to joules!

The KE=PE(when water is at the bottom or impact)!

English units are important too, unfortunately, but very real.

Anyone what to double check me here? :mellow:

7. Apr 19, 2010

### collinsmark

'Sounds good to me.