Laws of Mechanics:

Newton's three laws form a part of foundation of mechanics. These are:

First law: Every object in a state of uniform motion tends to remain in that state motion unless an external force is applied to it.

This law is also called “law of inertia”.

Second law: The change of motion is proportional to the natural force impressed and is made in a direction of the straight line in which the force is impressed.

Mathematically, this law is stated as:

F = m a

where F is the applied force, m is the mass and a is the acceleration. Note, that force and acceleration are vectors and are indicated in boldface letters.

Third law : For every action, there is an equal and opposite reaction .

The first two laws of Newton are valid only in inertial frame of reference. An inertial frame of reference has a constant velocity. That is, it is moving at a constant straight line.