Characteristics of momentum conservation law
Momentum is a vector. The equation of conservation of momentum is a vector equation. Generally, after specifying the positive direction, the physical quantity that can determine the direction will always indicate the direction as "+" or "-". Only the size is substituted into the physical quantity: the physical quantity that cannot be determined can be represented by letters. If the calculation result is "+", then explain The direction is the same as the specified positive direction. If the calculation result is "-", it means that the direction is opposite to the specified positive direction.
Momentum is an instantaneous quantity, and the law of conservation of momentum refers to the momentum and constant at any instant of the system. Therefore, the law of conservation of momentum listed is m1v1+m2v2+...=m1v1ˊ+m2v2ˊ+..., where v1, v2... are the instantaneous speeds at the same time before the action, and v1ˊ and v2ˊ are the instantaneous speeds at the same time after the action. As long as the system satisfies the condition of conservation of momentum, at any instant in the interaction process, the total momentum of the system is conserved. In specific problems, we can list conservation expressions of momentum based on the momentum of each object in any two instantaneous systems.
The momentum of an object is related to the choice of reference frame. Usually, the ground is taken as the reference frame, so the speed before and after the action must be relative to the ground.
It is applicable not only to systems composed of two objects, but also to systems composed of multiple objects; not only to systems composed of macroscopic objects, but also to systems composed of microscopic particles.