Newton's laws of motion

             

              Newton's Laws of Motion 

   Newton has given three laws to describe the motion of bodies. These laws are known as Newton's Laws of Motion. The Newton's laws of motion give a precise definition of force  and establish a relationship between the force applied on a body and the state of motion acquired by it. We will now  discuss these laws of Motion and consider some of their important applications. Let us  start with the first law of motion.

           Newton's first law of motion

Some of the bodies (or objects) around us are at rest, that is, they are stationary,  whereas others are in motion. Newton's first law describes the behaviour of such  bodies which are in a state of rest or of uniform motion in a straight line. According to Newton's first law of motion : A body at rest will remain at rest, and a body in motion will continue in motion in a straight line with a uniform speed, unless it is compelled by an external force to change its state of rest or of uniform motion.

         Newton's second law of motion

 When two bodies, a heavy one and a light one, are acted upon by the same force for the same time, the light body attains a higher velocity (or higher speed ) then the heavy one. But the momentum gained by both the bodies is the same. The link between force and Momentum is expressed in Newton's second law of motion.

            According to Newton's second law of motion : The rate of change of momentum of a body is directly proportional to the applied force, and takes place in the direction in which the force acts.The rate of change of momentum of a body can be obtained by dividing the 'Change in momentum' by  'Time taken' for change. So, Newton's second law of motion can be expressed as : 

                    Force = Mass × Acceleration 

             The Use of Seat Belts in Cars

These days all the cars are provided with seat belts for passengers to prevent injuries in case of an accident. In a car accident, a fast running car stops suddenly. Due to this the car's large Momentum is reduced to zero in a very short time. The slightly stretchable seat belts worn by the passengers of the car increased the time taken by the passengers to fall forward. Due to longer time, the rate of change of momentum of passengers is reduced and hence less stopping force act on them. So, the passengers may either  not get injured at all or may get less injuries. It is obvious  that seat belt reduce the passenger's momentum more gently and hence prevent injuries.

            Newton's third law of motion

When a body influences another body by applying force, we say that the first body is interacting with the second body. In any interaction between two bodies, there always two forces that come into play. And Newton's third law of motion describes the relationship between the forces that come into play when the two bodies interact with one another. 

According to Newton's third law of motion : Whenever one body exerts a force on another body, the second body exerts an equal and opposite force on the first body. 

                   The force exerted by the first body on the second body is known as "action" and the force exerted by the second body on the first body is known as "reaction". It  should be noted that "action" and "reactions" are just forces. We can now  write another definition of Newton's third law of motion : To every action there is an equal and opposite reaction. Action (force) and reaction (forces) act on two different bodies, but they act simultaneously.