Can momentum be zero? If yes, when?

Yes, momentum can be zero when an object is at rest or when the total momentum of a system is balanced.

Momentum is a vector quantity, which means it has both magnitude (size) and direction. The momentum of an object is calculated by multiplying its mass by its velocity. Therefore, if an object is at rest (i.e., its velocity is zero), then its momentum will also be zero, regardless of its mass. This is because any number multiplied by zero equals zero. For example, a stationary car has zero momentum because it is not moving, even though it has a substantial mass.

In addition, the total momentum of a system can also be zero. This happens when the system is in equilibrium, meaning the sum of all the momenta in one direction is equal to the sum of all the momenta in the opposite direction. This is often the case in collisions or explosions, where the total momentum before the event is equal to the total momentum after the event, a principle known as the conservation of momentum. For instance, if two identical billiard balls collide head-on with equal speed, they will come to a stop after the collision. Although each ball had momentum before the collision, the total momentum of the system is zero both before and after the collision because the momentum of one ball is exactly balanced by the momentum of the other.

In conclusion, momentum can indeed be zero, either when an object is at rest or when the total momentum of a system is balanced. Understanding this concept is crucial for solving problems in physics, particularly those involving collisions or other interactions between objects.