2023 MT Physics Short Questions

Question 1

An object is at rest on a tabletop. Earth pulls downward on this object with a force equal in magnitude to mg. If this force serves as the action force, what is the reaction force in the action–reaction pair?

A. Earth pushing upward on the table
B. the object pulling upward on Earth
C. the table pushing up on the object
D. the table pushing down on Earth
E. the object pushing down on the table

Solutions

Understanding Newton's Third Law

Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction.

The key to correctly identifying the reaction force is to remember two crucial points:

Equal and Opposite: The reaction force must have the same magnitude and act in the opposite direction to the action force.
Different Objects: The action and reaction forces always act on different objects. If the action force is ${\vec{F}}_{A on B}$ (force of object A on object B), the reaction force must be ${\vec{F}}_{B on A}$ (force of object B on object A).

Applying the Law to Your Problem

Your action force is:

{\vec{F}}_{Earth on Object}

(The force by the Earth on the object - acting downward)

Therefore, the reaction force must be:

{\vec{F}}_{Object on Earth}

(The force by the object on the Earth - acting upward)

The object is pulling upward on the Earth with a force of the same magnitude, $m g$ .

Analyzing the Options

Let's look at your choices based on this understanding:

A. Earth pushing upward on the table: This involves Earth and the table, not the Earth and the object. Incorrect.

B. the object pulling upward on Earth: This is the force by the object on the Earth. This perfectly matches our definition for the reaction to ${\vec{F}}_{Earth on Object}$ . Correct.

C. the table pushing up on the object: This is the Normal Force acting on the object. It is an entirely different force that happens to balance the gravitational force acting on the object (since the object isn't accelerating vertically). It's a force pair with the object pushing down on the table, but it's not the reaction force to the Earth's pull on the object. Incorrect.

D. the table pushing down on Earth: This involves the table and Earth. Incorrect.

E. the object pushing down on the table: This is a component of a different action-reaction pair (the object and the table). Incorrect.

The correct choice is B. the object pulling upward on Earth.