Conservation law?

Question

Can anyone explain what a conservation law is?

Answer 1

matter cannot be created nor destroyed.

it means that you cannot take a something like a rock and completely destroy it, it always has the molecules left over even in microscopic levels.

also, you cannot just create atoms out of thin air.

they have always existed and always will.

Answer 2

And energy and momentum are conserved.

Energy can be converted from one form to another. Often some is converted into forms that are not useful, like waste heat. Careful analysis will be able to find where all the original energy ended up. But none of the original energy is destroyed. And you can't create energy.

Momentum in a system before an event will equal momentum in the system after an event. Like before and after a collision, if you calculate the momentum of all the pieces, before has to equal after.