Newtons Laws are empirical laws, deduced from experiments. They were clearly stated for the first time by sir Isaac Newton, who published them in 1687 in his famous book called “principia”. Newton’s laws are adequate for speeds that are low compared with the speed of light. For very fast moving objects, such as atomic particles in an accelerator, relativistic mechanics developed by Albert Einstein is applicable.

You have already studied these laws in your secondary school physics. However a summarized review is given below.

An unappreciated anticipation.

 No body begins to move or comes to rest of itself.

 ABU ALI SENA, (980-1037)

 Newton’s First Law of motion

A body at rest will remain at rest, and a body moving with uniform velocity will continue to do so, unless acted upon by some unbalanced external force. This is also known as law of inertia. The property of an object tending to maintain the state of rest or state of uniform motion is referred to as the object’s inertia. The more inertia, the stronger is this tendency in the presence of a force. Thus,

The mass of the object is a quantitative measure of its inertia.

The frame of reference in which Newtons first law of motion holds, is known as inertial frame of reference. A frame of reference stationed on earth is approximately an inertial frame of reference.

For your information


A measurement of mass independent of gravity. The unknown mass m and a calibrated mass mc are mounted on a light weight rod. If the masses are equal. The rod will rotate without wobble about its centre.

Newton’s Second Law of Motion

A force applied on a body produces acceleration in its own direction. The acceleration produced varies directly with the applied force and inversely with the mass of the body. Mathematically, it is expressed as

F = ma                   ………………..         (3.9)

Newton’s Third Law of Motion

Action and reaction are equal and opposite. For example, whenever an interaction occurs between two objects, each object exerts the same force on the other, but in the opposite direction and for the same length of time. Each force in action-reaction pair acts only on one of the two bodies, the action and reaction forces never act on the same body.

Point to Ponder

A car accelerates along a road. Which force actually moves the car?