SAT Subject Physics Practice Question 497: Answer and Explanation

Next steps

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


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 (Ff) opposing the motion of the block exactly equals the downward force (F11).

Previous       Next