If a man has a mass of 100kg on earth what will his mass on the moon be?

The question is ill-posed. Weight is mesured in Newtons (N) as it's a force, it's mass is measured in kg. There are ctually two questions mixed together here. Let's answer both: 1) If an astronaut has a mass of 100kg on earth what is his mass on the moon? 100kg - put him on a frictionless surface and try and accelerate him, it's just has hard on the moon as on earth (or anywhere else for that matter). 2) If an astronaut weighs 981N on the surface of the earth (as an astronaut of mass 100kg would) how much does he weigh on the moon? Surface gravity on the earth is 9.81m/s/s which is how we end up with the 100kg astronaut weighing 981N. On the moon surface gravity is only 1.62m/s/s so the same astronaut would weigh 162N - about 1 sixth that on earth.