Center of Mass:The center of mass is the average position of all the particles in a system. It behaves as if all the mass is concentrated at that point.
Momentum:Momentum is the product of an object's mass and its velocity. In systems of particles, momentum is conserved unless acted upon by external forces.
Collisions: Collisions occur when two or more objects interact with each other for a short period of time. They can be elastic (kinetic energy is conserved) or inelastic (kinetic energy is not conserved).