Drop a ball, and it bounces back up. But each bounce is a little lower than the one before, until the ball lies still on the floor. Where did its energy go? Did it disappear?

Scientists have checked this again and again, in thousands of experiments. The answer is always the same. Energy never disappears. It only moves somewhere else or changes into another form.

The rule that is never broken

This is one of the most important rules in all of science. It is called the conservation of energy. Here, “conservation” means that the amount is kept the same.

The rule has two parts:

  • Energy can never be made from nothing.
  • Energy can never be destroyed.

Energy can only move from one place to another, or change from one form into another. Add up all the energy before something happens and all the energy after it. You always get the same amount.

A swing: height and speed take turns

Sit on a swing and ask a friend to pull you back, up high. At the highest point, you stop for a moment. The energy of swinging is now all stored in your height.

Then you swoop down. Your height turns into speed, and at the lowest point you move fastest. Now the energy is movement energy.

On the other side, you climb up again and slow down. The speed turns back into height. Height, speed, height, speed: the energy keeps changing from one form into the other.

A swing turning height into speed and back A swing hangs from a bar. It is shown in three places: high on the left, at the bottom, and high on the right, a little lower than at the start. Under each place, a bar shows its energy. High up, the energy is stored in height. At the bottom, it is movement. A small part has turned into heat, and that part grows with every swing. All three bars have the same total size. high fast high height movement heat
At the top, the energy is stored in height. At the bottom, it has become movement. On every swing a little turns into heat, but the total stays the same.

If nobody pushes, though, the swing goes a little less high each time. In the end it stops. Did its energy disappear?

No! The chains rub against their hooks, and the swing pushes the air out of its way. Each time, a tiny bit of movement turns into heat. The hooks and the air get very slightly warmer.

When two things rub against each other, movement turns into heat. This rubbing is called friction.

You can feel it yourself: rub your hands together fast for ten seconds. Feel how warm they get? That warmth is movement energy that has turned into heat.

A bouncing ball

A bouncing ball tells the same story. As it falls, its height turns into speed. When it hits the floor, the ball squashes a little, like a spring. Then it springs back and flies up again.

But each bounce is lower. With every bump, some of the energy turns into heat in the ball and the floor. Some becomes the sound of the bounce, and the sound fades away into heat too.

Now add it all up: the heat, the sound and the energy left in the ball. Together they make exactly the amount of energy the ball had at the start.

Where does “used-up” energy go?

We often say that energy gets “used up”. A battery goes flat, and a car runs out of fuel. But the energy has not disappeared. It has changed into forms we cannot use any more, mostly heat.

Think about a torch. The battery’s stored energy becomes electricity, and then light. The light hits the walls and warms them a tiny bit. In the end, all the energy has become heat, spread thinly around the room.

Spread-out heat is very hard to use again. It is like sand from a sandcastle blown all over the beach. None of the sand is gone, but you cannot build a castle with it any more.

That is why we keep needing fresh, useful energy. Our bodies need food, cars need fuel, and homes need electricity from power stations.

Why no machine can run forever

For hundreds of years, inventors dreamed of a machine that would keep going forever without any push. Some even hoped it would make extra energy for free. Such a dream machine is called a perpetual motion machine. “Perpetual” means never-ending.

None of them has ever worked, and none ever will. A machine cannot make energy from nothing. And every real machine loses a little of its movement to friction, as heat. Without new energy, it slows down and stops.

The heat pump puzzle

Here is a puzzle. A heat pump can warm a house with about three times as much heat as the electricity it uses. Does it break the rule? Does it make energy from nothing?

No. Most of that heat does not come from the electricity at all. The heat pump uses electricity to move heat that is already there. Even cold winter air still has some heat in it, and the heat pump collects it and carries it indoors.

Count everything, and the rule still holds. The heat taken from outside, plus the electricity, adds up to the heat that goes into the house. Nothing is made from nothing.

Where the heat from a heat pump comes from One square of electricity goes into a heat pump from above. Two squares of heat come into it from the cold air outdoors. Three squares of heat come out of it into the house. Nothing is made from nothing: 2 plus 1 makes 3. electricity heat pump outdoors cold air indoors heat electricity
The heat pump uses 1 part of electricity to move 2 parts of heat from the cold air outside. 3 parts of heat go into the house.
Go deeper: The first law of thermodynamics for grown-ups and the extra curious

Thermodynamics is the science of heat and energy. Its first law is the conservation of energy, written for one object:

ΔU=Q+W\Delta U = Q + W

Here ΔU\Delta U is the change in the energy stored inside the object (its internal energy), QQ is the heat that flows into it, and WW is the work done on it, for example by squeezing or stirring it. All three are measured in joules (J).

Energy cannot vanish, but it can become less useful. Spread-out heat can never be turned back completely into movement. That is what the second law of thermodynamics says (see heat).

The rule has passed every test, from swings to stars. In nuclear reactions even mass counts as energy, following Einstein’s famous E=mc2E = m c^2: energy equals mass times the speed of light squared. With that included, the total still stays the same.

Go deeper: Joule's paddle wheel for grown-ups and the extra curious

In the 1840s, James Prescott Joule showed that heat and movement are two forms of the same thing. He let a falling weight turn a paddle wheel inside a closed tank of water. The paddles churned the water, and the water got a tiny bit warmer.

Joule measured very carefully. The same amount of falling always gave the same amount of warming: about 4.2 joules of movement warm 1 gram of water by 1 °C. Before Joule, many scientists thought heat was an invisible fluid called “caloric”. His experiments helped to show that heat is a form of energy. The unit of energy, the joule, is named after him.

Go deeper: Counting a heat pump's energy for grown-ups and the extra curious

Engineers describe a heat pump with its coefficient of performance (COP): the heat it delivers divided by the electricity it uses. A COP of 3 means 3 kWh of heat for 1 kWh of electricity.

The extra 2 kWh are not created. They are taken from the outdoor air, the ground or a lake or river. The colder it is outside, the harder the heat pump has to work, and the lower its COP.

Go deeper: Where the water picture breaks down for grown-ups and the extra curious
  • Water is a substance you can see and touch. Energy is not: it is a quantity, a number that can be calculated and that nature keeps constant.
  • Spilled water can be mopped up and poured back into the jug. Spread-out heat never gathers back into one place by itself. To collect it again, you need a machine that uses more energy.

Check yourself

Why does a swing slowly stop if nobody pushes it?

Show the answer

Its movement slowly turns into heat, by rubbing and by pushing the air — The energy does not vanish. Friction in the hooks and the air slowly turn the swing's movement into heat.

Check yourself

A heat pump gives a house three times as much heat as the electricity it uses. How is that possible?

Show the answer

It moves heat from the outside air into the house — Even cold air has some heat in it. The heat pump carries that heat indoors. Heat from outside plus the electricity add up to the heat the house gets.