Newton's third law states that for every action there is an equal and opposite reaction.

Inherent property of all the bodies by virtue of which they cannot change their state of rest or uniform motion along a straight line by their own is called inertia.
Inertia of rest: It is the inability of a body to change by itself, its state of rest. This means a body at rest remains at rest and cannot start moving by its own.
(i) When a horse starts suddenly, the rider tends to fall backward on account of inertia of rest of upper part of the body as explained above.
(ii) Passengers experience a backward push due to inertia of rest.
Inertia of motion: It is the inability of a body to change itself its state of uniform motion i.e., a body in uniform motion can neither accelerate nor retard by its own.
(i) Passengers experience a forward push due to inertia of motion.
(ii) A person jumping out of a moving train may fall forward.
Inertia of direction: It is the inability of a body to change by itself direction of motion.
(i) A stone moves tangential to circle due to inertia of direction.
(ii) When a car goes round a curve suddenly, the person sitting inside is thrown outwards.

The centrifugal force appears to act on the particle, only when we analyses the motion from a rotating frame. With respect to an inertial frame there is only centripetal force which is given by the tension in the string. For this reason centrifugal force is called as a 'pseudo force'. A pseudo force has no origin, It arises due to the non-inertial nature of the frame considered.

One Newton is that force which produces an acceleration of 1 m/s^-2 in a body of mass 1 kilogram.
∴ 1 Newton = 1 kg m/s²