Chapter 11, Problem 34A

(a)

To determine

The potential energy of earth − ball system.

Explanation of Solution

Introduction:

The potential energy of the earth − ball system depends on the separating distance between them which is the work done in separating the earth and ball.

The chosen reference level is top which leads one to believe that initial potential energy of the ball is zero. And this potential energy is decreasing until the ball hits the ground.

The reference level chosen by the friend is bottom, therefore, he would believe that there is positive initial potential energy which is decreasing until the ball reaches zero at the ground level.

Conclusion:

Hence, the energy calculated by using these two-reference level would be different.

(b)

To determine

The change in the system’s potential energy as a result of the fall.

Explanation of Solution

Introduction:

Change in the potential energy = Final potential energy − Initial potential energy

Distance fallen by the ball according to both reference level would be same, therefore the change in the potential energy of the system as a result of the fall in both reference level would be same.

Conclusion:

Hence, the change in the system’s potential energy as a result of the fall would be same.

(c)

To determine

The kinetics energy of the system at any point.

Explanation of Solution

According to one, the ball is moving away from the reference point which would cause loss in kinetic energy and gain in potential energy.

According to one’s friend, the ball is moving towards the reference point which would cause the loss in potential energy and gain in kinetic energy.

Conclusion:

Hence, the kinetic energy at any point according to two deferent level would be different.