Question

An object of mass m rests on a flat table. The earth pulls on this object with a force of magnitude mg. What is the reaction force to this pull?

Answer 1

The reaction force to Earth's gravitational pull on an object is the normal force, which has the same magnitude as the object's weight but in the opposite direction, thereby allowing the object to remain at rest on a table.

Step-by-step explanation:

The reaction force to the pull of Earth on an object with mass m is known as the normal force. According to Newton's third law of motion, for every action, there is an equal and opposite reaction. Therefore, if the Earth is pulling on the object with a force of mg, where g is the acceleration due to gravity (approximately 9.80 m/s² on Earth), then the table must be pushing up on the object with an equal force. This upward force is the normal force exerted by the table on the object, and it has the same magnitude as the weight of the object but in the opposite direction. Thus, the reaction force is mg directed upward. This concept is also evident when we consider the object's weight—the gravitational force on a mass m—which is calculated using the formula F = ma = mg. If there were no reaction force, the object would not remain at rest on the table.

Answer 2

The reaction force of the object on the flat table will be in upward direction with the same magnitude mg.

Step-by-step explanation:

According to third law of motion, every action has equal and opposite reactions. So here, the action is the gravitational pull acting downward on the object kept on table with a magnitude of mg.

So as per third law, the reaction of the object will be in opposite direction to the action i.e., the pull will be in the upward direction as reaction to the gravitational pull toward downward direction and the magnitude should be same as mg.

Thus, the reaction exerted by the object on the table for the action of gravitational force of magnitude will be the upward pull of the object from the table with the magnitude mg and as both the action and reaction will be canceling each other, the object will remain at the same position on the table without any motion as there is no unbalanced force in the system.