Calculate the gravitational force between the Earth and the moon. The mass of the Earth 5.97 × 1024 kilograms and the mass of the moon is 7.36 × 1022 kilograms. The radius of the orbit is 382,171 km.
F = [G x Me x Mm] / R^2
F = [6.6742 E-11 x 5.97 x 10^24 x 7.36 x 10^22] / 382171000^2 [you must convert km to meters]
F = 2.00787E+20 Newtons
P.S. I have assumed that the radius of the orbit is from center to center of the two bodies.
