Energy: Making Things Happen

Kick a ball, and it flies. Wind up a toy, and it scoots across the floor. Eat breakfast, and you can run around all morning. Every single thing that happens — every movement, every sound, every flash of light — needs energy to make it go.

What energy is

Energy is the universe’s “make things happen” stuff. You can’t hold a lump of it, but you can watch it move things, heat things, light things up, and make noise. No energy, no action — a universe without energy would just sit there, perfectly still, forever.

The two big kinds

Energy wears lots of costumes, but two show up everywhere:

• Moving energy (scientists say kinetic energy) — the energy something has because it’s moving. A rolling ball, a running dog, the wind. Faster and heavier means more of it.
• Stored energy (scientists say potential energy) — energy that’s saved up, ready to go. A ball held above the ground. A stretched rubber band. The food in your lunchbox and the charge in a battery are stored energy too — chemistry holding energy until you need it.

Here’s the connection to gravity: lifting something stores energy in it. Carry a ball to the top of a slide and you’ve done the saving-up. Let go, and gravity spends those savings — the stored energy turns into moving energy as the ball speeds down.

The golden rule

Now for one of the biggest rules in all of science:

Energy never disappears, and it never comes from nowhere. It only changes from one costume into another.

Scientists call this the conservation of energy, and no experiment ever done — anywhere, by anyone — has caught energy breaking it. A rollercoaster is the rule in action: the chain lifts the car high (storing energy), then the whole ride is just that one pile of energy sloshing back and forth — fast at the bottoms, slow at the tops — until the brakes turn what’s left into heat.

Play with energy

Time to see the golden rule with your own eyes. There’s a ball on a skate-park track, and three bars keeping count of the energy.

• Drag the ball up a hill and let go. Watch the amber stored bar drain into the teal moving bar on the way down, then fill back up on the way up. The total never changes.
• Try to beat the middle hump. Release the ball from below the hump’s height — it can never, ever get over. Release it from higher up — over it goes.
• Switch friction on. Now the red heat bar creeps up and the ball slowly settles. The energy didn’t vanish — it turned into heat.

Where the energy goes

So if energy never disappears… why does a swing stop swinging when you stop pumping? Why does a rolling ball slow down?

Friction. Every rub — wheels on a ramp, air brushing past a swing — steals a little moving energy and turns it into heat, spread out and too thin to gather back up. Rub your hands together fast and you can feel it happening: moving energy becoming warm hands. The energy is all still there, it’s just worn its most forgettable costume.

Work and power

Two quick words scientists use about energy:

• Work is what it’s called when energy gets handed from one thing to another. Lift your school bag onto a hook and you did work on the bag — some of your breakfast’s energy is now stored in the hanging bag.
• Power is how fast the handover happens. Walking upstairs and sprinting upstairs hand over the same energy — the sprint just does it faster. More power, same work.

Key words

Check yourself

You’ve now got the second key to black holes: escaping gravity is about having enough energy. Next up is the third and strangest key — light, the fastest thing in the universe, and the speed limit that nothing can ever break.