Laws of Motion

First Law of Motion


" When an object is moving uniformly on a straight path or is at rest, its state would not change until and unless an external force is applied on it ".


Hence an objects oppose a change in their state of motion or rest. This tendency of an object wants to remain in the state of rest or to keep moving uniformly is called Inertia. Ist law of motion is also known as Law of Inertia.


The factor on which inertia depends:

  • The inertia of an object is dependent upon on the mass of that object.
  • Lighter objects have less inertia, that is, they can easily change their state of rest or motion.
  • Heavier objects have large inertia and therefore they show more resistance.
  • Therefore ‘Mass’ is called a measure of the inertia of an object.


Second law of motion:

Statement :-

" The rate of change of momentum of an object is directly proportional to the applied unbalanced force on the object in the direction of the force".


A karate player makes use of the second law of motion to perform the task of breaking a slab of bricks.


Momentum :

  • The product of mass and velocity is called Momentum.
  • It is a vector quantity. Its direction is the same as that of the object’s velocity.
  • Denoted by – p
  • SI unit – kg metre per second
  • p = mv, where m is the mass of the object and v is the velocity of the object


 Mathematical formula for 2nd law of motion:

Suppose an object of mass, m is moving along a straight line with an initial velocity, u. It is uniformly accelerated to velocity, v in time, t by the application of a constant force, F throughout the time, t.

 The initial and final momentum of the object will be, p1 = mu and p2= mv respectively.

  • F is the force
  • k is the constant of proportionality
  • a is the acceleration

The unit of force is kg m s-2 or newton, which has the symbol N.

Third law of motion:

  • When an object exerts a force on another object, the second object instantly exerts a force back onto the first object. These forces are always equal in magnitude but opposite in direction. These forces always act on two different objects.
  • Statement “Every action has an equal and opposite reaction"
  • The magnitudes of forces acting upon the objects are the same but the acceleration produced in them may or may not be the same because the objects can differ in masses.


Conservation of Momentum:

As per the law of conservation of momentum, the sum of momenta of two objects before the collision and after collision remains the same given that no external unbalanced force acts upon them. In another way, collision conserves the total momentum of two objects.