A ball sits at rest on a horizontal table top.

The gravitational force on the ball (its weight) is one half of an action-reaction pair. Which force is the other half?

Question

A ball sits at rest on a horizontal table top.

The gravitational force on the ball (its weight) is one half of an action-reaction pair. Which force is the other half?

BlackTom AI · Accepted Answer

Here is a step-by-step analysis of each option.

Option 1: 'the upward force that the ball exerts on the Earth' — This is the correct action–reaction partner of the ball's weight. The ball experiences a downward gravitational force from the Earth (weight). By Newton's third law, the ball exerts an equal and opposite force on the Earth. Since the Earth pulls the ball downward and the ball pulls the Earth upward, these two forces form an action–reaction pair. The directions and bodies involved are consistent with the third-law pairing.

Option 2: 'the upward force that the table top exerts on the ball' — This is the normal contact force between the ball and the table. While this force acts upward on the ball, it is not the reaction to the ball's weight. The weight is due to Earth's gravity, and its reaction is the Earth experiencing an upward pull from the ball. The normal force is part of the contact interaction between ball and table, not the weight–earth pair.

Option 3: 'the force of the Earth’s gravity on the ball' — This describes the weight itself, i.e., the downward force the Earth exerts on the ball. It is the cause of the weight, not the reaction to it. The third-law partner to this force is the ball’s upward force on the Earth, not another force on the ball.

Option 4: 'the downward force that the ball exerts on the table top' — This is the ball’s action on the table via contact. It is indeed related to the table–ball interaction, but it is not the action–reaction pair to the ball’s weight. The corresponding reaction to the ball’s downward push on the table is the table’s upward normal force on the ball, not a downward force from the ball.

In summary, the only option that correctly identifies the action–reaction partner to the ball’s weight (the Earth’s gravitational pull on the ball) is the upward force that the ball exerts on the Earth, reflecting Newton’s third law.

SPHY001 Graded Quiz 2 (Unit 3 and Unit 4)- Requires Respondus LockDown Browser

A ball sits at rest on a horizontal table top. The gravitational force on the ball (its weight) is one half of an action-reaction pair. Which force is the other half?

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