Why Can't Energy Be Created or Destroyed?

Let's start with the three major conservation laws.

These are the VIPs of physics that never break as long as nothing from the outside interferes.

The first is conservation of energy.

Energy cannot be created out of nothing nor destroyed to nothingness.

It can only change form.

For example, chemical energy in fuel turns into kinetic energy of a car, which turns into heat, and so on.

But if you add it all up, the total energy stays the same.

This rule, the first law of thermodynamics, is absolutely fundamental.

The second is conservation of momentum.

There's linear and angular.

Both are important, but for simplicity, I'll talk about linear momentum.

Just know that the same principle applies to the total spin of a system.

Momentum is the product of mass times velocity.

You can think of it as how much something is moving.

Heavier things moving faster have more momentum than lighter things moving slower.

In an isolated system, the total momentum remains constant.

When playing billiards, for example, when the cue ball hits object balls, its momentum is transferred.

If you added up the mass times velocity of all the balls before and after the collision, it would remain the same.

The third is conservation of electric charge.

The total charge of a system remains the same.

Charge can be transferred but not created or destroyed.

So for example, in a chemical reaction, when one molecule gains a negative charge due to the transfer of an electron, the other molecule gains a positive charge.

Total charge remains the same in an isolated system.

You can't create charge out of nothing.

These are the big ones, energy, momentum, and charge.