The first law of motion is normally taken as the definition of inertia. If there is no net force acting on an object then object will remain a constant velocity. If an external force is applied, the velocity of body will change because of force.
This law explains how velocity of an object changes when it is subjected to an external force. The law defines force to be equal to change in momentum (mass times velocity) per unit time.
For an object with constant mass m, Newton’s second law of motion states that the force ‘F’ is the product of an object’s mass ‘m’ and its acceleration ‘a’.
F = m.a
For an externally applied force, the acceleration depends on mass of the object and a change in velocity will generate a force. The above equation works in both ways.
In other words, if object ‘A’ exerts a force on object ‘B’, then object ‘B’ also exerts an equal force on object ‘A’.
The third law of motion can be used to explain the generation of lift by a wing and the production of thrust by a jet engine.