When a bus moving along a straight line takes a turn to the right, the passengers are thrown towards left. This is due to inertia which makes the passengers travel along the same straight line, even though the bus has turned towards the right.
This inability of a body to change by itself its state of rest or of uniform motion along a straight line or direction, is known as inertia. The inertia of a body is directly proportional to the mass of the body.
From the first law, we infer that to change the state of rest or uniform motion, an external agency called, the force is required.
Force is defined as that which when acting on a body changes or tends to change the state of rest or of uniform motion of the body along a straight line.
A force is a push or pull upon an object, resulting the change of state of a body. Whenever there is an interaction between two objects, there is a force acting on each other. When the interaction ceases, the two objects no longer experience a force. Forces exist only as a result of an interaction.
There are two broad categories of forces between the objects, contact forces and non-contact forces resulting from action at a distance.
Contact forces are forces in which the two interacting objects are physically in contact with each other.
Tensional force, normal force, force due to air resistance, applied forces and frictional forces are examples of contact forces.
Action-at-a-distance forces (non- contact forces) are forces in which the two interacting objects are not in physical contact which each other, but are able to exert a push or pull despite the physical separation.
Gravitational force, electrical force and magnetic force are examples of non- contact forces.