Chapter 4.3, Problem 34SSC

To determine

The forces acting on the ball and its interaction pair.

Explanation of Solution

Introduction:

Newton’s Third Law of Motion:

According to Newton’s third law, for every actionthere is an equal and opposite reaction. When a body $A$ applies a force on the body $B$ then the body $B$ exerts an equal and opposite force on the body $A$ .

  $F_{\text{A on B}}=-F_{\text{B on A}}$

Consider a person is holding a motionless ball in his hand.

The forces will act on the motionless ball are as follows:

1. The gravitational force produced as a result of the interaction between the mass of the ball and the mass of the Earth. The gravitational force will act in the downward direction.

2. The force exerted by the hand on the motionless ball in the upward direction.

Sketch the forces acting on the motionless ball as shown below:

  

Refer to the Figure.

The force exerted by hand on the ball $\left(F_{\text{hand on ball}}\right)$ and the force exerted by the Earth’s mass on the ball $\left(F_{\text{Earth's mass on ball}}\right)$ forms the interaction pair (also known as action-reaction pair) as these forces are equal in magnitude and act in opposite directions.

Conclusion:

The two forces acting on the motionless ball are the gravitational force (acting in the downward direction) and the force exerted by the hand on the ball (acting in the upward direction).

The Earth’s gravitational force on the ball and the force exerted by the hand on the ball forms an interaction pair.