Question

When the body is placed on a tabletop, it exerts a force equal to its weight downwards on the tabletop but does not move or fall.
(i) Name the force exerted on the tabletop
(ii) What is the direction of the force.

Answer 1

Here, we use the concept of normal force and Newton’s third law of motion. The normal force is in the essence of Newton’s third law. Newton’s third law state to every action there is an equal and opposite reaction. And normal means to perpendicular force away from the surface that an object is at rest.

Complete step by step answer:
i) We know the normal force that surfaces exert to prevent solid objects passing through each other. It is also called a contact force. If two surfaces are not in contact, they do not exert a normal force on each other. When two surfaces are in contact, they exert a normal force that is perpendicular to the contacting surfaces.
To justify the statement, the force exerted on the tabletop. Consider a book(body) lying on the table; if you are pushing down on the book with force, this also acts as the weight of the book. The table would push at the book with (normal force) equal to the sum of the weight of the book and the force applied by you. Therefore both are pushing each other. This is the case of action and reaction, i.e. Newton’s third law of motion and weight of the book is equal to the normal reaction force.
So the force exerted on the table of the top is the normal force.

ii) Since the direction of the force is opposite to each other hence no resultant action occurs.

Note:
Since the normal force is reactive, its magnitude is independent of the nature of the force causing it. The most common normal force is caused by gravity. There can be additional forces that cause normal force.