MCAT Physics Question 7: Answer and Explanation
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Question: 7
7. A 10 kg wagon rests on a frictionless inclined plane. The plane makes an angle of 30° with the horizontal. Approximately how large is the force required to keep the wagon from sliding down the plane?
- A. 10 N
- B. 49 N
- C. 85 N
- D. 98 N
Correct Answer: B
Explanation:
The static force of friction acts parallel to the plane and is in the opposite direction from the parallel component of gravity in this setup. Because the wagon is in equilibrium, these two forces are equal in magnitude. Remember that gravity is often split into components in inclined plane problems. Rather than splitting into x- and y-components, however, it is more convenient to split the gravity vector into parallel and perpendicular components. The parallel component of gravity is given by the expression mg sin θ. Plugging in the values from the question, both the parallel component of gravity and static force of friction must be equal to 
(sin 30°) = 49 N.