What does it mean if momentum is conserved in a system?

When momentum is conserved in a system, it indicates that the total momentum of the system before an event, such as a collision, is equal to the total momentum after the event. This principle is grounded in the law of conservation of momentum, which states that in a closed system where no external forces are acting, the total momentum remains constant.

This concept is fundamental in understanding interactions such as collisions and explosions in physics. For example, when two objects collide, the momentum they share before the collision is redistributed between them after the collision, but the total remains the same. This understanding helps in analyzing various scenarios in both classical and modern physics, such as in vehicle collisions, sports, and even astrophysical events.

The other options don't accurately describe the concept of momentum conservation. Momentum doesn't double for individual objects, nor does the system gain energy from elsewhere, nor does momentum decrease over time. Instead, it's the totality of momentum in the defined system that provides a consistent outcome before and after an event.