Where Does the Energy Go?

Last lesson we left a question hanging.

You kicked a ball. It rolled, friction slowed it, and it stopped. We said friction did the stopping — and that's true. But it doesn't answer everything.

The ball had something while it was moving. Call it whatever you like: go, oomph, movement. When the ball stopped, that something was gone.

So where did it go?

It's a fair question and it has a genuinely surprising answer: it didn't go anywhere. It's still here, in the field, in the ball, in the air. You just can't see it any more, because it turned into something you can only feel.

That something is called energy, and following it around is one of the most powerful things a scientist ever learns to do.

By the end of this lesson you'll be able to:

• say what energy is and name its main forms
• follow energy through a chain as it changes from one form into another
• explain why energy is never used up, only moved and spread out
• tell renewable energy sources from non-renewable ones

Energy is what lets things happen

Here's the working definition, and at your level it's enough:

Energy is what a thing needs in order to make something happen.

Nothing happens without it. A ball won't move, a bulb won't light, a fan won't turn, you won't be able to stand up. Every one of those needs energy, and energy has to come from somewhere.

It shows up in different forms, and the main ones are worth knowing by name:

• Movement energy — anything that is moving has it. A rolling ball, running water, wind.
• Stored energy — energy that isn't doing anything yet but could. Food and fuel store it chemically; a coconut up a tree stores it by being high up; a stretched rubber band stores it too.
• Heat energy — in anything warm.
• Light energy — from the Sun, a fire, a bulb.
• Sound energy — in anything you can hear.
• Electrical energy — flowing through wires.

Now the rule that makes all of this useful, and it's the biggest idea in the lesson:

Energy is never created and never destroyed. It only changes from one form into another.

Read that again, because it means something strict. When your torch battery dies, no energy was destroyed. When the ball stopped, none was lost. The total amount in the universe right now is the same as it was yesterday and the same as it will be tomorrow. All that ever happens is that energy moves around and changes costume.

So "where did the ball's energy go?" is always answerable. You just have to follow it.