How much work does the earth do on the moon in one day?

[jazz07]

Assume that the Earth and the moon have circular orbits (which is almost true) and that the periods of their orbits are 365 and 28.0 , respectively.

How much work does the Earth do on the moon in one day?
How much work does the sun do on the Earth in one month?

Im totally lost on this problem...i tried using the equation G=(m,earth)(m,moon)/r^2 but that doesn't work! any advice on how to sove this question?

 

[Sourabh N]

W = F . r (since F is constant)
Whats the angle betw F and r