Newton, the father of physics established the laws of motion in his book "principia" in 1687.

Every body maintains its initial state of rest or motion with uniform speed on a straight line unless an external force acts on it.

» First law is also called law of Galileo or law of inertia.

» **Inertia** : Inertia is the property of a body by virtue of which the body opposes change in its initial state of rest or motion with uniform speed on a straight line.

» Inertia is of two type - Inertia of rest and Inertia of motion.

» First law gives the definition of force.

**Force**

Force is that external cause which when acts on a body changes or tries to change the initial state of rest or motion with uniform velocity of the body.

**Momentum**

Momentum is the property of a moving body and is defined as the product of mass and velocity of the body. i.e. *momentum = mass * velocity*.

» It is a vector quantity. Its SI unit is kgm/s.

The rate of change in momentum of a body is directly proportional to the applied force on the body and takes place in the direction of force.

**F = m a**

Where, F = force applied, a = acceleration produced and m = mass of body

» Newton’s second law gives the magnitude of force.

» Newton’s first law is contained in the second law.

To every action, there is an equal and opposite reaction.

If no external force acts on a system of bodies, the total linear momentum of the system of bodies remains constant

As a consequence, the total momentum of bodies before and after collision remains the same.

» Rocket works on the principle of conservation of linear momentum.