**Newton’s first law of motion **states that a body will continue in its state of rest or uniform motion in a straight line unless an external resultant force acts on it.

Newton’s First Law of Motion implies that

- The state of rest requires no resultant force to maintain.
- The state of uniform velocity also requires no resultant force to maintain.

**Inertia** is the tendency of an object to continue in its original state of motion in absence of a resultant force.

Every material object has inertia and the amount of matter present in the object.

- Mass is the quantity of matter in an object and is independent of the body’s surroundings and method used to measure it (scalar quantity)
- Inertial mass m
_{i}specifies how much inertia an object has. It is a measure of the response of an object to an external force. - Gravitational mass, m
_{g}is determined by measuring the force on an object in a gravitational field (i.e. its weight)

**Newton’s second law **states that the rate of change of momentum of a body is proportional to the resultant force acting on it and the change takes place in the direction of the force.

$F = ma + v \frac{dm}{dt}$

Change in momentum = ma

If m is constant, F = ma

If m is not constant, then $F = v \frac{dm}{dt}$

Newton’s second law describes the change of motion when a non-zero net force is applied on object.

The amount of momentum, which an object has, depends on its mass and velocity

The linear momentum, p, of a body is the product of its mass, m, and its velocity, v of a moving body.

$P = mv$

It is a vector quantity and its direction follows the direction of the velocity. The SI unit of momentum is $\text{kg m s}^{-1}$ or $\text{N s}$.

The more momentum an object has, the harder it is to stop and the greater effect or impact it will experience if it is brought to rest.

**Newton’s third law **states that: If body A exerts a force on body B, then body B exerts a force of equal magnitude but in the opposite direction on body A.

**Note:**

- It does not matter which force is called action and which is called reaction
- The two forces must be of the same type i.e. if one is an electrical force, then the other must be electrical
- The two forces act on different bodies thus they do not produce zero resultant force (do not cancel each other) on any one object.