First law of motion: An object continues to be in a state of rest or of uniform motion along a straight line unless acted upon by an unbalanced force.
The natural tendency of objects to resist a change in their state of rest or of uniform motion is called inertia. The mass of an object is a measure of its inertia. Its SI unit is kilogram (kg).
Force of friction always opposes motion of objects.
Second law of motion: The rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.
The SI unit of force is kg m s–2. This is also known as newton and represented by the symbol N. A force of one newton produces an acceleration of 1 m s–2 on an object of mass 1 kg.
The momentum of an object is the product of its mass and velocity and has the same direction as that of the velocity. Its SI unit is kg m s–1.
Third law of motion: To every action, there is an equal and opposite reaction and they act on two different bodies.
In an isolated system (where there is no external force), the total momentum remains conserved.