Newton’s law of motion Sir Isaac Newton ( in 1687 ) formulated the fundamental laws of mechanics. These axiomatic laws can be stated as follows.

First law: A body continues in its state of rest or constant velocity unless acted upon by an external force.

Second law: The rate of change of momentum is proportional to the impressed force.

The momentum is defined as the product of mass and velocity.

F = dp/dt

if the mass m is a constant,

F = m [dv/dt] = m a

The SI unit force is newton (N).

Third law: Whenever two bodies interact, the force on the second body due to the first, F₂₁ is equal and opposite to the force on the first due to the second, F₁₂.

i.e. F₁₂ = - F₂₁

The first law gives the meaning of force, while the second law gives the quantitative definition of force.

Sometimes the mass of a body is not constant, as in the case of a satellite ejecting fuel, or if a body travels with speed comparable to the speed of light. In these situations eq.(n4) defines the true force that acts on the system. It is also necessary that reference system should be nonaccelerating, for the first and second law to hold.

There is an inherent limitation of the validity of third law as all signals travel with finite velocity. F₂₁ and F₁₂ are equal if they are measured at the same instant of time. The third law may not be valid when duration of collision is too small, because of the finite time interval required for the forces to propagate.