# when does normal force come into play?is the friction of air the normal force applied by air on the object that is moving in it?

Aarti Gupta
9 years ago
The normal force comes into play any time two bodies are in direct contact with each other.It always acts perpendicular to the body that applies the force. The simplest example of the normal force- is the situation of a man standing on a platform.In this case gravitational force acts on the man, pulling him down, perpendicular to the platform; but since the man is not moving, another force must act to counteract the gravitational force. This force is applied by the platform, and is called the normal force, and is referred to as FN .The normal force can also be taken as a direct consequence of Newton's Third Law.In the situation of the man standing on the platform, his weight, due to the gravitational force, pushes down on the platform. Newton's third law predicts that this force on the platform must be accompanied with an equal and opposite force applied to the man by the platform. This force is the normal force. Since the normal force is a reactive force, its magnitude is independent of the nature of the force causing it.
When the object is moving in air, a frictional force acting in the opposite direction to the movement of the object and thus opposes the motion of an object.It is not an normal force.