1. The problem statement, all variables and given/known data earth mass is approximately 81 times the mass of the moon. if earth exerts a gravitational force of magnitude f on the moon, the magnitude of the gravitional force of the moon on earth is 1) F 2) f/81 3) 9f 4) 81 F 2. Relevant equations i think the equation should be 81f 3. The attempt at a solution is it right that gravity*mass gives magnitude?