SAT Subject Physics Practice Question 15: Answer and Explanation
Assuming a frictionless, massless pulley, determine the acceleration of the blocks once they are released from rest.
Correct Answer: E
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:
FT – mg = ma (1)
Mg – FT = Ma (2)
Adding these equations yields Mg – mg = ma + Ma = (M + m)a, so