   Chapter 4, Problem 20CQ

Chapter
Section
Textbook Problem

A large crate of mass m is placed on the back of a truck but not tied down. As the truck accelerates forward with an acceleration a, the crate remains at rest relative to the truck. What force causes the crate to accelerate forward? (a) the normal force (b) the force of gravity (c) the force of friction between the crate and the floor of the truck (d) the “ma” force (e) none of these

To determine
The force that causes the crate to accelerate forward.

Explanation

Friction force opposes the relative lateral motion when the surfaces of two objects are in contact. The direction of friction force is parallel to the surfaces. It can either be static or kinetic, depending upon the state of rest or motion.

Conclusion:

As the truck accelerates forward, friction opposes the relative motion between the truck and the crate. Thus, friction force is along the horizontal direction and causes the acceleration of the crate. Thus, option (c) is correct.

Normal force acts along the vertical direction...

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