A ball is dropped from one metre and bounces back to 60 centimetres. What happened to the rest?

At one metre it had its full potential energy. After the bounce it has only enough for 60 centimetres. The difference was transferred during the impact. About 40 percent went into warming the ball and floor, into deformation, and into sound

Draw the energy chain for a lit pocket torch.

The source is chemical energy in the cell. In the wires it becomes electrical energy. At the bulb it becomes light and heat. Chemical → electrical → light + heat

At which point is a pendulum moving fastest, and why?

At the lowest point the height is smallest. Potential energy shrinks as height shrinks. Whatever potential is missing has become kinetic. At the bottom, because that is where kinetic energy is greatest

Energy and Heat · Grade 8

Where does the energy go when a bouncing ball rises lower each time?

Nowhere — it is all still there. On every impact some energy goes into deforming the ball, into warming the rubber and the floor, and into the sound of the bump. The ball rises less because it has less mechanical energy left, but the total has not changed at all. It has simply spread into forms that are hard to see.

Learning objectives

Draw an energy chain for a device such as a torch or a bicycle and label every conversion along it
State the law of conservation of energy and apply it to a ball that bounces lower each time
Answer the question of where the energy went whenever it appears to have been lost
Explain why a swinging pendulum is fastest at the bottom and slowest at the ends of its swing

The law itself

The law of conservation of energy says that energy is never created and never destroyed. It only changes form and changes place. The total in a closed system stays exactly constant.

This is among the best-tested laws in all of physics, and no violation has ever been found. Every time energy appeared to have gone missing, careful measurement showed it had moved into a form nobody had measured — almost always heat.

That gives you a working rule. When energy looks as though it has disappeared, do not assume it has. Look for what got warmer, what made a noise and what got bent.

Drawing an energy chain

An energy chain is an ordered list of the forms the energy passes through, with an arrow between each pair. For a torch: chemical in the cell → electrical in the wires → light and heat at the bulb.

For a bicycle: chemical in food → kinetic in rider and bike → thermal in friction and brakes. For a computer: electrical → light at the screen, sound at the speaker, and heat at the processor.

Notice that heat appears at the end of nearly every chain. That is not a flaw in your drawing; it is a property of the world, and without it the efficiency of lesson six makes no sense.

A pendulum: two forms trading places

A pendulum released from the side starts with potential energy alone. As it falls, its height drops and its speed rises — potential is turning into kinetic.

At the lowest point the height is least and the speed is greatest, so nearly all the energy is kinetic. Swinging up the other side reverses the trade, and the pendulum pauses for an instant when every joule is potential again.

With no friction it would reach exactly the same height forever. In practice it stops — not because energy vanished, but because friction at the pivot and against the air moves it into heat at a slow, steady rate.

Worked examples

A ball is dropped from one metre and bounces back to 60 centimetres. What happened to the rest?
1. At one metre it had its full potential energy
2. After the bounce it has only enough for 60 centimetres
3. The difference was transferred during the impact
Answer: About 40 percent went into warming the ball and floor, into deformation, and into sound
Draw the energy chain for a lit pocket torch.
1. The source is chemical energy in the cell
2. In the wires it becomes electrical energy
3. At the bulb it becomes light and heat
Answer: Chemical → electrical → light + heat
At which point is a pendulum moving fastest, and why?
1. At the lowest point the height is smallest
2. Potential energy shrinks as height shrinks
3. Whatever potential is missing has become kinetic
Answer: At the bottom, because that is where kinetic energy is greatest

Common mistakes

Saying the energy was lost: Energy is never lost. It has moved into a form nobody looked for, and in most everyday cases that form is heat spread thinly through the surroundings, which is exactly why it is hard to measure.
Confusing conservation of energy with conserving energy: The law says the amount never changes; conservation in the everyday sense is about keeping energy in a useful form. They are two quite different meanings of one word.
Expecting a pendulum to swing forever: The law guarantees the total is conserved, not that it stays mechanical. Friction moves it into heat bit by bit, so the swing dies away even though nothing was destroyed.

What to remember

Energy changes form; the total is conserved.
"It disappeared" almost always means heat.
An energy chain names every single conversion.
A pendulum trades potential for kinetic and back.