Newtons laws of motion: 

• First law: Law of inertia: Every body continues to be in its state of rest or of uniform motion in a straight line unless compelled to change that state by an external unbalanced force.
• Second law: The rate of change of momentum is directly proportional to the external unbalanced force and takes place in the direction of force. 
• Third law: To every action, there is an equal and opposite reaction.

Force and Inertia:

Force: Force is that which changes or tends to change the state of rest or of uniform motion of a body in a straight line.

For Eg:

(i) A body remaining at rest cannot be moved without the help of an external force.

(ii) A freely rolling body will come to rest due to frictional force. So, in order to change the state of a body, we need a force. So a body by itself cannot change its state of rest or of uniform motion in a straight line. This property of all bodies is called inertia.

Inertia: Inertia is the inability of a material body to change by itself its state of rest or of uniform motion in a straight line.