What principle states that the total momentum before and after a collision remains constant?

The principle that states that the total momentum before and after a collision remains constant is known as the Law of Conservation of Momentum. This law is fundamental in mechanics and is based on the idea that in an isolated system, where no external forces are acting, the total momentum remains unchanged.

When two or more objects collide, they exert forces on each other. The momentum of each object may change due to the collision, but the total momentum—defined as the product of mass and velocity for all objects involved—will remain the same before and after the collision. This principle applies to all types of collisions, including elastic and inelastic collisions, and is crucial for analyzing interactions in a variety of physical scenarios.

The other principles mentioned, such as the Law of Conservation of Energy, Newton's First Law, and the Law of Action and Reaction, pertain to different aspects of physics. The conservation of energy deals with the transformation of energy types, Newton's First Law refers to inertia and the idea that an object in motion will remain in motion unless acted upon by an outside force, and the Law of Action and Reaction describes how forces act in pairs. While these laws are important, they do not specifically describe momentum in the context of collisions.