We all know that a heavy truck is harder to stop than a small car moving at the same speed. We state this fact by saying that the truck has more momentum than the car. By momentum we mean inertia in motion or, to be more specific, the product of the mass an object and its velocity:

Momentum= mass*velocity
Or in shorthand notation
Momentum=mv

We can see from the definition that a moving object can have large momentum if either its mass or its speed is large, or if both are. A truck has more momentum than a car moving at the same speed because the truck has a greater mass. A huge ship at a low speed has a large momentum because of its huge mass . A massive truck with no brakes rolling down a steep hill has a large momentum, whereas the same truck at rest has no momentum at all because the v part of the mv is zero.

Momentum is the product between mass and velocity. Being a vector quantity, it has a direction, and the direction is very important when doing momentum calculations. Momentum is not a thing that we can see, but it does explain many things that go on in physics.

Momentum (kg m/s) = mass (kg) x velocity (m/s)

Conservation of Momentum

An important principle:
The total momentum of a system remains constant provided that no
external forces act on the system.

This has important implications in the study of collisions. In simple terms, we can say that the total momentum before = total momentum after. The key thing is that share of the momentum may change.

Collisions

Momentum is always conserved in collisions.
If objects bounce off each other, the collision is elastic. If the total kinetic energy is the same (conserved) at the end as it is at the start, then the collision is perfectly elastic. The rebound of particles against each other tends to be perfectly elastic. A tennis ball bouncing off the floor is not perfectly elastic as it can lose up to 25 % of its kinetic energy in doing so.
If some kinetic energy is lost, converted into heat or light, then the collision is inelastic.