Conservation of Momentum: Introduction

Momentum conservation requires two conditions: the system's total mass must remain constant during interaction, and the net external force acting on the system must be zero. When both conditions are met, the system is called a closed or isolated system, and total momentum remains unchanged regardless of internal particle interactions.

Internal forces between particles are always equal in magnitude and opposite in direction, following Newton's third law. The momentum changes caused by each internal force are canceled by equal and opposite momentum changes from the reaction force. Therefore, internal forces sum to zero and cannot alter the system's total momentum.

Initially, only the rolling ball has momentum while pins are at rest. When collision occurs, the ball slows down and pins gain momentum. The total momentum of the entire system remains constant because momentum lost by the ball equals momentum gained by the pins, illustrating that total momentum is conserved in the closed system.

Yes, external forces can act on a system as long as they cancel each other, resulting in zero net external force. For example, billiard balls experience weight and normal forces, but these balance perfectly. When external forces cancel completely, the system remains closed and momentum conservation applies.

When net external force is nonzero, the system is no longer closed and momentum is not conserved. External forces change the total momentum of the system. This is why momentum conservation only applies to closed systems where all external forces sum to zero.

Mass can transfer between objects during interaction, but total system mass remains constant because any mass one object gains is balanced by loss from another. Since total mass is unchanged and net external force is zero, momentum conservation still applies to the system as a whole.

In a closed system, momentum before interaction equals momentum after interaction. The law of conservation of momentum states that the change in total momentum due to particle interactions is zero. This principle holds at all scales, from galactic clusters to subatomic particles.