Newton’s Laws Of Motion

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

  1. The state of rest requires no resultant force to maintain.
  2. 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 mi specifies how much inertia an object has. It is a measure of the response of an object to an external force.
  • Gravitational mass, mg 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.


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


Back To Forces and Dynamics (A Level)

Back To A Level Topic List

Sharing is caring:
Mini Physics


1 thought on “Newton’s Laws Of Motion”

Leave a Comment