First Law of Motion:
It states “A body remains at rest or continue to move with uniform velocity in a straight line, unless some unbalanced force acts upon it to change its state of rest or uniform motion”
Inertia:
The property of a body to oppose any change in its state of rest or of uniform motion is called inertia.
Newton’s Second Law of Motion:
“When a force is applied on a body, it produces acceleration in the body. The acceleration is in the direction of the force. The acceleration is directly proportional to the applied force and inversely proportional to the mass of the body.”
Mathematically F=ma
Force:
Force is an agent which when applied to a body changes or tends to change its state of rest, or of uniform motion in a straight line.
Unit of force:
In S.I system the unit of force is newton.
F = ma
1N= 1kg. 1m s-2It is defined as the force which produces an acceleration of 1m s-2 in a body of mass 1kg.
Inertial Mass:
Mass is the quantity of matter in a body which remains constant every where in the universe.
From Newton’s second law of motion 
Thus, mass determines the acceleration produced in a body. The larger the mass the smaller will be the acceleration for a fixed applied force
Newton’s Third Law of Motion:
When two bodies interact the action is always equal and opposite to reaction.
“To every action force there is an equal and opposite reaction force.”
It means that action always acts on one body and reaction on the other body. That is why action and reaction never balanced each other.
If two bodies A and B collide, the action of body A on B is force 
and reaction of B on A is force 
Then according to Newton’s third law of motion we can write
.
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