Question

An object is at rest on the ground. The object experiences a downward gravitational force from Earth. Which of the following predictions is correct about why the object does not accelerate downward? Select two answers. Justify your selections.

A) The bonded molecules of the object are repelled upward by the bonded molecules of the ground with the same magnitude as the gravitational force downward on the object.

B)The normal force is exerted upward on the object from the ground with the same magnitude as the gravitational force downward on the object

C) The bonded molecules of the object are attracted downward by the bonded molecules of the ground with the same magnitude as the gravitational force downward on the object.

D) The force of friction is exerted upward on the object from the ground with the same magnitude as the gravitational force downward on the object.

Answer 1

The correct answers are:

1. A) The bonded molecules of the object are repelled upward by the bonded molecules of the ground with the same magnitude as the gravitational force downward on the object.
2. B)The normal force is exerted upward on the object from the ground with the same magnitude as the gravitational force downward on the object.

EXPLANATION:

According to Newton's third law of motion, For every action there will be an equal and opposite reactions.

An object placed on ground will experience a gravitational force acting downwards, directed towards the center of the earth. Hence, the bonded molecules of the ground will experience some compressing force which these molecules tends to oppose. From Newton's third law of motion, These molecules will exert an opposite force to the bonded molecules of the object with same magnitude. Hence the forces get cancelled each other and the object will remain stationary.

Similarly, the net force exerted by the ground on the object as a reaction is called Normal force. This normal force will be equal and opposite to the action (which is the gravitational force) and get balanced.

Note: Force of friction between two surfaces will always act in a direction of motion. Here the force of friction is irrelevant unless the object is placed in some sort of friction pit or something and tends to move down.