A body continues to be in its state of rest or of uniform motion along a straight line unless it is acted upon by some external force to change the state.
If no net force acts on a body, then the velocity of the body cannot change i.e. the body cannot accelerate.
Newton’s first law defines inertia and is rightly called the law of inertia.
Newton’s Second Law:
The rate of change of linear momentum of a body is directly proportional to the external force applied on the body and this change takes place always in the direction of the applied force.
If a body of mass m moves with velocity then its linear momentum can be given by = and if force is applied on a body, then
∴ = Where F = Force, m = mass and a = acceleration
Newton’s Third Law:
To every action, there is always an equal (in magnitude) and opposite (in direction) reaction.
When a body exerts a force on any other body, the second body also exerts an equal and opposite force on the first.
Forces in nature always occur in pairs. A single isolated force is not possible.
Explanation:
Newton's second law of motion states that the force is equal to the change in momentum per change in time. It also equals mass times acceleration.