# Gravitational force (I think )

1. Oct 2, 2007

### map7s

1. The problem statement, all variables and given/known data

A student of mass 72 kg is at an altitude of 1.5 km falling towards the Earth's surface, accelerating at 9.8 m/s2. What is the acceleration of the Earth as it falls toward the student? The radius of the Earth is 6.38 x 10^6 m.

2. Relevant equations

3. The attempt at a solution

I have no idea how to go about solving this problem. I don't know what to do with the information that is given or even what equations to use. (but I do know that my answer has to have an exponent of 10^-23 m/s2)

2. Oct 2, 2007

### Staff: Mentor

TWO masses exert a force on each other, i.e. F = G M m /r2.

The m is subjected to an acceleration of g = G M / r2, so can one determine the acceleration of M?

Last edited: Oct 2, 2007
3. Oct 2, 2007

### map7s

oh...okay...that makes more sense....well G is a constant and M is given....so for r, should I use the radius of the earth? (because I was thinking to use the distance that the person was falling b/c that would be the "radius" between the two objects....but then what would I do with the radius of the earth?....and does M correspond to the mass of the earth or the mass of the person?)

4. Oct 2, 2007

### map7s

is there any other way to do this without using the gravitational force equation?

5. Oct 2, 2007

### learningphysics

There is... but I'm guessing they want you to solve it using the gravitational force equation because they give you the radius of the earth.

The earth experience a net force of GMm/r^2 (here r is not the radius of the earth, but the distance from the center of mass of the earth to the falling person... ie the radius of the earth + the altitude), where M is the earth's mass. So what is the acceleration of the earth?