Chapter 4, Problem 2Q

A box rests on the (frictionless) bed of a truck. The truck driver starts the truck and accelerates forward. The box immediately starts to slide toward the rear of the truck bed. Discuss the motion of the box, in terms of Newton’s laws, as seen (a) by Mary standing on the ground beside the truck, and (b) by Chris who is riding on the truck (Fig.4-35).

To determine

a)Motion of the box, in terms of Newton’s laws as observed by Mary standing on the ground beside the truck

b)Chris who is riding the truck.

Answer to Problem 2Q

Solution:

a)According to Newton’s first law,an unbalanced external force is needed to change the state of motion of a body. In this given problem, there is no force acting on the box. Thus, the box should remain in its place.

b)According to Newton’s second law, when a force acts on the mass an acceleration is produced. Chris observes that the box is accelerating backward towards the rear of the truck.

Explanation of Solution

Given:

A box rests on the (frictionless)bed of a truck. the truck driver starts the truck and accelerates forward. The box immediately starts to slide toward the rear of the truck bed.

Applying Newton’s laws of motion here:

Newton’s 1st Law: Everybody continues in its state of rest or uniform motion unless it is compelled by an unbalanced external force

Newton’s 2nd Law: The force acting on the mass accelerates the object.

a)In this given problem, there is no force is acting on the box. Thus the box should remain in its place. Mary observes that box stay stationary with respect to the ground. There is no force acting on the box since the truck bed is smooth, and so the box can’t move. Therefore Mary will see from the ground that box will remain stationary and only truck accelerating and moves forward.

(b)According to Chris sitting in the truck, he will observe that the box is accelerating backward towards the rear of the truck. Chris, from his non-inertial reference frame, would see the box being “thrown” backward in the truck. Using Newton’s 2nd law to describe the effects of that force.