For any collision occurring in an isolated system, momentum is conserved. The total amount of momentum of the collection of objects in the system is the same before the collision as after the collision. A common physics lab involves the dropping of a brick upon a cart in motion.

How do you know when momentum is conserved?

The momentum of an object will never change if it is left alone. If the 'm' value and the 'v' value remain the same, the momentum value will be constant. The momentum of an object, or set of objects (system), remains the same if it is left alone. Within such a system, momentum is said to be conserved.

In which case momentum is conserved?

During projectile motion and where air resistance is negligible, momentum is conserved in the horizontal direction because horizontal forces are zero. Conservation of momentum applies only when the net external force is zero. The conservation of momentum principle is valid when considering systems of particles.