Newton, the father of Physics established the laws of motion in his book “Principia” in 1687.
- First Law: Every body maintains its initial state of rest or motion with uniform speed on a straight line unless an external force acts on it. This law is also called ‘Law of Galileo (or) Law of Inertia’. This law gives definition of Force.
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 2 types: (i) Inertia of Rest; (ii) Inertia of Motion.
Examples: a) When a Car or Train starts suddenly, the passenger bends backwards.
- b) When a running horse stops suddenly, the rider bends backward.
- c) When a coat/blanket is beaten by a stick, the dust particles are removed.
- Second Law: 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.
Force is that external cause which acts on a body changes or tries to change the initial state of the body.
Momentum is the property of a moving body and is defined as the product of mass and velocity of the body i.e Momentum (P) = Mass (m) × Velocity (v). It is a vector quantity. Its SI unit is kgm/s.
If F = force applied, a = acceleration produced and m = mass of body, then nF = ma.
Newton’s second law gives the magnitude of force. Thus first law is contained in the second law.
- Third Law: To every action, there is an equal and opposite reaction.
Examples: (i) Recoil of a gun; (ii) Motion of Rocket; (iii) Swimming; (iv) While drawing water from the well, if string breaks up, water falls back.