# Homework Help: Help, Rotational Motion

1. Jan 14, 2008

### jgens

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

Calculate the centripetal acceleration of the Earth in its orbit around the sun and the net force exerted on the Earth. What exerts this force on the Earth? Assume that the Earth's orbit is a circle of radius (r) 1.5 * 10^11 m.

2. Relevant equations

Centripetal acceleration = (v^2)/r = ((4)(pi^2)(r))/(t^2)

Centipetal Force = ((m)(v^2))/r

3. The attempt at a solution

Centripetal acceleration = ((4)(pi^2)(1.5 * 10^11 m))/(31536000^2 s^2) = .006 (m)/(s^2)

I'm ok with the centripetal acceleration but I don't know how to find the average net force without the mass. The centripetal force in this case is the gravitational force of the sun pulling on the Earth. Can someone please give me an idea on how to solve this problem? Thanks.

2. Jan 14, 2008

### Dick

You do need the mass of the earth to compute the force. Since they didn't give it to you, you'll have to look it up.

3. Jan 15, 2008

### jgens

All right. Thanks.