# Gravitational potential energy.

1. Nov 23, 2008

### cruisx

1. The problem statement, all variables and given/known data
Hey guys, just wondering if you guys could help me out a bit.

A boy on a sled with a total mass of 72kg slides from rest down a hill, which is 35m high. The friction force between sled and snow has a magnitude of 250N. If the slope of the hill makes an angle of 37 degrees with the horizontal, find:
a) the gravitational potential energy of the sled at the top of the hill relative to the base.

2. Relevant equations

If i am not mistaken the equation for Gav.pot energy is
U= GMe * m
.........Re2

and the value of g can be found as g = G * Me/ (R + h)2

3. The attempt at a solution

I know i have the following from the question:

m= 72kg
h1=35m
Ff= 250N

now what i dont get is where the 37 degrees is supposed to come in and i am not quite sure that that is the equation that i am supposed to use. Could someone please explain to me how i would begin this question? I want to solve it my slef but can someone point me in to a direction. thank you.

edit: wait a second, i think i was doing it wrong. Would it be like E1g = mgh = (72kg)(9.8m/s2)(35m) = 24696 J ?

Last edited: Nov 23, 2008
2. Nov 23, 2008

### Staff: Mentor

Much simpler would be to use PE = mgh, which is perfectly good near the Earth's surface.

Maybe you don't need it for part a.

3. Nov 23, 2008

### Staff: Mentor

Now you've got it.

4. Nov 23, 2008

### cruisx

Ok cool thanks guys, i think i can handle it form here ^_^. look like i will be studying all day for that physics test tomorrow

5. Nov 23, 2008

### cruisx

Sorry for posting again but now i am stuck at part c of the question which is : the speed of the sled at the bottom of the hill.

How would i find this out? I know i have to use the 37 degrees somehow but i am stuck, I now have the following variables.

m=72kg
h1 = 35m
Ff = 250N
gav.pot energy : 24696J
Work: 8750J

Its been a while since i have had to find speed so a hint would be helpful =)

6. Nov 23, 2008

### Staff: Mentor

What's the work done by friction? Hint: What's the distance traveled by the sled?

Set up an energy equation.