# Conservation of Energy and calculating potential energy?

1. Dec 11, 2005

### suspenc3

during a rockslide a 520kg rock slides from rest down a hillside that is 500m long and 300m high. The coeeficient of kinetic friction is 0.25. If the gravitational potential energy $$U$$ of the rock-earth system is zero at the bottom of the hill, what is the value of $$U$$ just beofre it slides?

I know this is wrong, but I tried:

$$U_g = mghcos \Theta$$
$$U_g = 9.2 x 10^5J$$

What do I do?

2. Dec 11, 2005

### Staff: Mentor

This is a good question. There is too much information given in the problem. Specifically, neither the length of the hillside nor the coefficient of friction are important. The gravitational potential energy is just U = mgh. The statement about U = 0 at the bottom of the hill just establishes that h = 0 at the bottom of the hill (since m and g are never 0).

The important part is: do you understand why the friction and length are unimportant in this question?

-Dale

Last edited: Dec 11, 2005
3. Dec 11, 2005

### suspenc3

well friction wouldnt matter because it only has to do with thermal energy?

but there are more parts to the question, that is why alot of the info seems useless.

Why doesnt the angle come into play when calculating potential energy?

4. Dec 11, 2005

### naeblis

Potential energy is the energy a mass has at rest, potentially it could fall and turn that energy into kinetic energy. change in U is = change in K [conservation of energy] U doesnt even care about any angles because there is no angle involved with a rock sitting in one place.

5. Dec 11, 2005

### Staff: Mentor

That and also gravity is a conservative field. Meaning that the path you take (the angle) is irrelevant, only the change in the potential is important.

Suspenc3, you hit the key idea about the friction. Basically, if it were frictionless than all of the PE at the top would go into KE at the bottom. With the friction some of the PE will go to heat and some will go to KE, but the initial PE is still the same.

-Dale

6. Dec 11, 2005

### gunblaze

Ya..Other information are irrelevent.
You can simply use $$mgh$$ to find the change in GPE, then add it to zero.. U shld get an answer.