A body at rest will remain in rest, and a body in motion will continue in motion in a straight line with uniform speed, unless it is compelled by an external force to change its state of rest or of uniform motion.
INERTIA: Inertia is that property of a body due to which it resists a change in its state of rest or of uniform motion.
Whenever one body exerts a force on another body, the second body exerts equal and opposite force to the first body. OR,
To every action there is equal and opposite reaction.
EX: the rocket works on the principal of action and reaction.
When a bullet is fired from the gun, a force sending the bullet forward is equal to the force sending the gun backward. But due to high mass of the gun, it moves only a little distance backward and gives jerk.