First, the momentum is conserved, and the external force on the system is zero;

Theorem: A system is free from external force or the sum of external forces is zero, and the total momentum of this system remains unchanged. This conclusion is called the law of conservation of momentum.

The second is the conservation of mechanical energy, and the system is only affected by gravity;

Theorem: Kinetic energy and potential energy (including gravitational potential energy and elastic potential energy) of an object system are transformed into each other, but the total amount of mechanical energy remains unchanged. This law is called the law of conservation of mechanical energy.

Third, the angular momentum is conserved, and the external force acting on the system is zero, that is, the resultant torque is zero;

Theorem: If the resultant torque is zero (that is, m =0), then L 1=L2, that is, L= constant vector. That is to say, for a fixed point o, the resultant torque acting on the particle is zero. The angular momentum vector of this particle remains unchanged. This conclusion is called the law of conservation of angular momentum of particles.