Law of conservation of momentum
The law of conservation of momentum states that p = const. applies in a closed system. Impulse can neither be generated nor lost, just like the energy in the Conservation of energy or the mass in the Conservation of mass. If you take a closer look at that, it means that the speed is also maintained beyond the speed conservation law described by me. So you can see that if m = const. applies and if p = mv = const. is that v = const. applies. Because v is p / m and an arbitrary but certain number divided by an arbitrary but certain number, is again an arbitrary but certain number, just const. in the physical sense. This would never have occurred to me, since the electron / positron annihilation would be a counterexample, if only photons arise from electron and positron. These photons are naturally faster than the electrons and positrons. In the meantime, however, you have probably reached the point where this process also creates mesons and the like, which means that there is again the possibility of maintaining the average speed.
The conservation of momentum also results in the conservation of the rest energy and thus the rest mass. The equation is from Einstein and reads: E² = E (0) ² + c²p². If one now sees, due to the Conservation of energy, that E is constant in the closed system and due to the conservation of momentum the term c²p² is also constant, then only E (0) can be retained. If, however, E (0) remains, the rest mass according to m (0) = E (0) / c² is also a conserved quantity.