Momentum and...a law from the physics

Conservation

A Newton's cradle demonstrates conservation of momentum.

In a closed system (one that does not exchange any matter with its surroundings and is not acted on by external forces) the total momentum is constant. This fact, known as the law of conservation of momentum, is implied by Newton's laws of motion.^[5] Suppose, for example, that two particles interact. Because of the third law, the forces between them are equal and opposite. If the particles are numbered 1 and 2, the second law states that F₁ = dp₁/dt and F₂ = dp₂/dt. Therefore

or

If the velocities of the particles are u₁ and u₂ before the interaction, and afterwards they are v₁ and v₂, then

This law holds no matter how complicated the force is between particles. Similarly, if there are several particles, the momentum exchanged between each pair of particles adds up to zero, so the total change in momentum is zero. This conservation law applies to all interactions, including collisions and separations caused by explosive forces.^[5] It can also be generalized to situations where Newton's laws do not hold, for example in the theory of relativity and in electrodynamics.^[6]