**AP Physics 1 Question 206: Answer and Explanation**

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**Question: 206**

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

- A.
- B.
- C.
- D.

**Correct Answer:** D

**Explanation:**

**D**

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 (D). Draw a free-body diagram:

Newton's Second Law gives the following two equations:

*F*_{T} - *mg* = *ma* (1)

*F*_{T} - *Mg* = *M(-a)* (2)

Subtracting these equations yields *Mg* - *mg* = *ma* + *Ma* = (*M* + *m*)*a*, so