# Universal gravitation - radius?

Using F = G * (m1*m2)/(r2)

We know that

G = 6.6673x10-11 N*m2/kg2
m1(earth) = 6x1024 kg
m2(moon) = 7.4x1022 kg
the earth and moon are an average of 3.9x108m apart

would r2 be [3.9x108]2 or [3.9x108 / 2]2?

-Thanks!

## Answers and Replies

Doc Al
Mentor
Why would you divide the distance in half?

Here we are using radius...........!
So, the first one is correct.

Here the distance between the two bodies mean the distances from their centres.
So, definitely, they are mentioning the radius