SAT Subject Physics Practice Question 167: Answer and Explanation
13. An astronaut standing on the surface of the moon (mass = M, radius = R) holds a feather (mass = m) in one hand and a hammer (mass = 100m) in the other hand, both at the same height above the surface. If he releases them simultaneously, what is the acceleration of the hammer?
Correct Answer: B
The gravitational force on an object of mass m can be expressed either by mg or by . Setting mg equal to , we get g = , which is the object’s (free-fall) acceleration. Notice that the mass of the object cancels out, so whether we’re asked for the acceleration of the feather or the hammer, the answer would be the same. (By the way, the experiment described in this question was actually performed by Apollo 15 astronaut David Scott on July 30, 1971. Both the feather and the hammer hit the lunar surface at the same time, verifying the fact—first stated by Galileo—that under co