Question

A child tosses a ball straight up. She says that the ball is moving away from her hand because the ball feels an upward “force of the throw” as well as the gravitational force. (a) Can the “force of the throw” exceed the gravitational force? How would the ball move if it did? (b) Can the “force of the throw” be equal in magnitude to the gravitational force? Explain. (c) What strength can accurately be attributed to the “force of the throw”? Explain. (d) Why does the ball move away from the child’s hand?

Answer 1

Many individuals have a misconception that throwing a ball in the air gives the ball some kind of a “force of motion” that the ball carries after it leaves the hand. This is the “force of the throw” that is mentioned in the problem. The upward motion of the ball is explained by saying that the “force of the throw” exceeds the gravitational force—of course, this explanation confuses upward velocity with downward acceleration—the hand applies a force on the ball only while they are in contact; once the ball leaves the hand, the hand no longer has any influence on the ball’s motion. The only property of the ball that it carries from its interaction with the hand is the initial upward velocity imparted to it by the thrower. Once the ball leaves the hand, the only force on the ball is the gravitational force.

(a) If there were a “force of the throw” felt by the ball after it leaves the hand and the force exceeded the gravitational force, the ball would accelerate upward, not downward!

(b) If the “force of the throw” equaled the gravitational force, the ball would move upward with a constant velocity, rather than slowing down and coming back down!

(c) The magnitude is zero because there is no “force of the throw.”

(d) The ball moves away from the hand because the hand imparts a velocity to the ball and then the hand stops moving.