I'm looking at a textbook problem which has an answer but no working out. I've read the chapter several times and looked at the equations but can't see where to start with it? Can you help? Assuming the Earth and Moon to be isolated from all other masses, use the following data to estimate the mass of the Moon. Mass of the Earth = 6.0 x 10^24 kg Distance between centre of Earth and centre of Moon = 3.8 x 10^8 m Distance from centre of Earth at which gravitational field is zero = 2.8 x 10^8 m Answer = 7.4 x 10^22 kg I think it's the 2 distances throwing me, Why do I need to know when gravity is zero? Do I need to find force? Help! Many thanks.