SAT Subject Physics Practice Question 15: Answer and Explanation

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Question: 15

6.

Assuming a frictionless, massless pulley, determine the acceleration of the blocks once they are released from rest.

A.
B.
C.
D.
E.

Correct Answer: E

Explanation:

One way to attack this question is to notice that if the two masses happen to be equal, that is, if M = m, then the blocks won’t accelerate (because their weights balance). The only expression given that becomes zero when M = m is the one given in E. If we draw a free-body diagram,

Newton’s second law gives us the following two equations:

FTmg = ma (1)
MgFT = Ma (2)

Adding these equations yields Mgmg = ma + Ma = (M + m)a, so

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