From Newton's Third Law we derive the idea of impulse and the Law of Momentum.

Take the example of the skaters.

Action and reaction

The force on one is the same as the force on the other and they push against each other for exactly the same amount of time, so

F x t is the same for each (although in opposite directions).

The term F x t is called the impulse.

Now from Newton's Second Law F = m x a

and rearranging this acceleration = change in velocity/ time i.e. a = (v-u)/t where v is the final velocity and u the initial or starting velocity


so, if we consider just one of the skaters F = m(v-u)/t  or rearranging F x t = m x (v-u)  a very useful expression

The term mass x velocity is called the momentum.

Mass x change in velocity is the change in momentum

the boy and girl are not the same size and are not likely to move back with the same velocity. If the boy has a mass M and velocity V and the girl mass m and velocity v

because the boy and girl and pushing against each other with the same force for the same time and the initial velocity u=0:

F x t = M x V = m x v in size (but not direction).

The total momentum of the two before they pushed was zero, because they were not moving. After they pushed apart mv and MV are equal but opposite, so they cancel each other out.

This gives rise to the Law of Momentum that:

The change of momentum in a collision (or an explosion) is zero. The total momentum in any closed system is constant.

Download a pdf on all the notes for Newton's Laws here  Newton's Laws of Motion

Related pages are Newton's First Law

Newton's second Law

Newton's Third Law

Impulse and Momentum