Question

explain with an example the internal force of a system do not contribute to the motion of its centre of mass. plz ans fast

Answer 1

with out mass the movementum will not occurs.

Answer 2

it happens due to the Newton's third law...
the net external force is always equal to the net internal force..
therefore, the momentum also becomes zero..
so, the internal force of a system do not contribute to the motion of its centre of mass..
for example:-
considered two billiards balls of mass m , one above the other falling under gravity. then the external force becomes 2 mg pointing down. now suppose we add a compressed springs in between the balls . this spring exerts a force f on top ball pointing up. and it must exert an opposite force on the bottom ball namely force f pointing down. therefore the total force is
mg- f (pointing down from the top ball) + mg + f (pointing down from the bottom ball). In the sum the force is cancelled out . so the total force is again 2 mg .
here you can see the internal force of a system do not contribute for the motion ...