The momentum of a particle of mass m is mv, where v is the velocity, so momentum is a vector quantity. Momentum is a measure of the inertia (resistance to velocity change) of a moving particle, just as the mass is a measure of inertia of a particle at rest. By using Newton 2 and 3, we show that if there is no resultant external force on a system (2 colliding spheres in this case), then the momentum of the system is conserved. Check out KeysToMaths1 for more tutorials and solutions: https://www.youtube.com/channel/UCOYoe9L9R13HGUIhszjkeug

The receding (relative) velocity of two spheres after colliding is proportional to the approaching (relative) velocity of the spheres just before the collision. Check out KeysToMaths1 for more tutorials and solutions: https://www.youtube.com/channel/UCOYoe9L9R13HGUIhszjkeug

For a particle colliding with a fixed object, the component of the final velocity perpendicular to the object v, is proportional to the component of the initial velocity perpendicular to the object u. v=-eu where e (coefficient of restitution) depends on the materials. Check out KeysToMaths1 for more tutorials and solutions: https://www.youtube.com/channel/UCOYoe9L9R13HGUIhszjkeug