A system is a certain number of objects under consideration. It can be only one object or it can be multiple bodies.
When we talk about a system we ignore the forces that are exerted by the particles/ bodies forming the system on each other. These forces are internal forces and using the third law they form action reaction pairs and hence cancel out as a whole.Therefore for such a system the Linear Momentum is Conserved if no external force acts on the system.
When we talk about a system we ignore the forces that are exerted by the particles/ bodies forming the system on each other. These forces are internal forces and using the third law they form action reaction pairs and hence cancel out as a whole.Therefore for such a system the Linear Momentum is Conserved if no external force acts on the system.