What stays conserved when two objects collide
In a collision, total momentum stays constant if external forces have negligible effect during the interaction. Momentum depends on mass and velocity, including direction. The objects may bounce apart or stick together, but their combined momentum before and after must match under that condition.
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Follow the whole system
Momentum is mass multiplied by velocity. Because velocity has direction, momenta in opposite directions can cancel when added.
During a collision, the objects exert equal and opposite forces on each other. Those internal exchanges change each object's momentum but not the total momentum of the pair. External forces must be negligible, or their effect must be included.
Momentum conservation does not mean kinetic energy is always conserved. Some kinetic energy may become heat, sound or deformation.
For example, two equal carts approaching at equal speeds have zero total momentum. If they stick together in an idealised collision with negligible external impulse, the joined pair is stationary.
- Individual momenta can change greatly.
- Total momentum remains the same under the stated condition.
Always specify the system before comparing totals.