EXPLANATION

If there is a collision between two bodies (with no external force), we know by newtons 3rd law that the force one exerts on the other will be equal and opposite to the force that the other exerts on it Hence by newtons sencond law they will both have the same rate of change of momentum. But one is in contact with the other for the same time as the other is in contact with it, so the change in momentum will be the same for both but in different directions, hence the vector sum of change of momentum will always be zero.

• This result can be extended to any number of particles.
• It is not neccessary for kinetic energy to be conserved, for the law of conservation of momentum to be applied.
• It is one of the most fundamental assumtions of A level physics, often needed in all sorts of questions.