# SAT Subject Physics Practice Question 497: Answer and Explanation

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**Question: 497**

**13.** A flat plane is raised until a block resting on its surface just slides down the plane at a constant rate. Which statement(s) is/are true?

I. The parallel force equals the frictional force.

II. The weight of the object equals the frictional force.

III. The perpendicular force equals the normal force.

A. I only

B. II only

C. I and III only

D. II and III only

E. I, II, and III

**Correct Answer:** A

**Explanation:**

The correct answer is (A). The force down the plane is a component of the weight of the block, which is found by multiplying the weight of the object by the sine θ of the angle to which the plane is raised. This produces a force down the plane, which will accelerate the object when the plane is raised high enough. The frictional force between the block and the plane *always* acts against the motion of the object-in this case, friction operates up the plane. Since there is no acceleration, the frictional force (F_{f}) opposing the motion of the block exactly equals the downward force (F_{11}).