# Homework Help: Potential energy.

1. May 11, 2010

### sphyics

1. When a body of mass 'm' is taken from the surface of the earth to a height equal to the radius of the earth (R) then the change in its Potential energy is______

got the solution as follows:
1) when on the surface of the earth the P.E is mgR (considered it as initial P.E)
when the body is taken to height 'h=R' frm the surface of earth the P.E is mg(R+R) = 2mgR (considered it as final P.E)
.. . change = Final P.E - Initial P.E = 2mgR-mgR= mgR

another approach:
2) on the surface of earth P.E = 0
when raised to "h=R" its P.E = mgR
.
. . change in P.E= mgR

out of these two approaches which is considered to be the best.

2. May 11, 2010

### zachzach

U = mgh is only a valid approximation when near Earth's surface. You must use the gravitation potential energy

3. May 11, 2010

### sphyics

can u make it more clear..

4. May 11, 2010

### zachzach

The gravitational potential energy is $$U = \frac{-GMm}{r}$$
where r is the distance from the center of mass of the Earth. You can use u = mgh as an approximation when close to Earth's surface.