Posted by **VitaX** on Saturday, October 16, 2010 at 9:17pm.

A 0.720 kg snowball is fired from a cliff 8.10 m high. The snowball's initial velocity is 14.2 m/s, directed 30.0° above the horizontal. (a) How much work is done on the snowball by the gravitational force during its flight to the flat ground below the cliff? (b) What is the change in the gravitational potential energy of the snowball-Earth system during the flight? (c) If that gravitational potential energy is taken to be zero at the height of the cliff, what is its value when the snowball reaches the ground?

I think I understand each problem. But I'll post what I think I need to do then someone can confirm if its right or not.

a)mgyi - mgyf = Wg I get 57 J

b)Change in Gravitational Potential Energy is just the negative of that work so its -57 J

c)The answer should just be zero since it was zero at the top of the cliff.

I'm skeptical because I didn't use the angle or velocity given. That's why I'm not sure if these answers are right or not. Do I use W = change in kinetic energy for part a or no?

- physics -
**bobpursley**, Saturday, October 16, 2010 at 9:49pm
c. If inital GPE is zero, then at the base of the clift it has to be -57J

- physics -
**VitaX**, Saturday, October 16, 2010 at 9:54pm
How can that be because the height at the ground is 0. So mgyf would be zero as well wouldn't it? How is my work for part a and b?

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