# Which is greater?

1. Mar 1, 2006

### atomant

Which, if either, is greater: the gravitational force of the sun on the Earth, or the Earth on the sun? How should I be looking at this question i.e in terms of the radius of each object through the equation F=GM1M2/r^2 ?

2. Mar 1, 2006

### pmb_phy

They're the same, i.e. wherever there is an action there is an equal and opposite reaction. You should look at the symmetry of F=GM1M2/r^2.

Pete

3. Mar 1, 2006

### atomant

So it's the same because the G is constant or because the r^2 is the same for bothe planets?

4. Mar 1, 2006

### Eivind

Newtons third law, F=-F.

5. Mar 1, 2006

### atomant

right, and if we were to calculate the the gravitational force exerted on the Earth by the sun we would use F=GMm/r^2.
If so I know G is constant, but what values do we substitute for the masses M and m? mass of earth and sun?

and if we were to find the gravitational force exerted on the sun by the Earth, how do the values change?

6. Mar 1, 2006

### Eivind

Yes, use GMm/r^2. For M you use the suns mass, and for m use the earths mass. r^2 is the distance sun-earth. The gravitational force exerted on the earth by the sun, is eqvivalent whit the force the earth exerts on the sun.
Let`s say F, is the force from sun to earth. Then -F is the force from earth to sun.