In physics in general, there are a few laws about various entities that always have to stay the same. One of these things that always stay the same is the electric charge. No matter what happens there will always be the same electric charge in the end as there was in the beginning. We say that the electric charge is conserved.

These laws of conservation are very important in physics. Many processes don't have to be taken into account because they violate one of the laws of conservation. The number of possible processes decreases and therefore it gets easier to classify a process. They can tell the scientist what is allowed to happen and therefore what he or she should look for. Take a look at the different reactions on the left. Can you find out which one violates the law of conservation of charge?

Other things that are conserved are energy, momentum and spin. Let's take a closer look at the conservation of momentum.

Momentum is defined as the mass (more or less the same thing as weight) of an object times its velocity (more or less speed). When two particles (or other things) collide, both of them have a momentum. What happens to their momenta when they collide? Try out some different masses and velocities in the animation below to find out.

Right! The total momentum never changes. No matter what happens, momentum is always conserved. When two objects collide, the individual momentum of the two may change, but the total momentum alway stays the same.

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