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

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