Try building your own Newton’s cradle, like the one on page 5. Use a shoebox, string, and 5 marbles. Tape a marble to the center of a piece of string long enough for the attached marble to dangle in the box without touching the bottom. Secure the two ends of string to opposite sides of the shoebox. Repeat for each of the other marbles so that the marbles are just touching.

Now raise a marble on one end and release it. The marble should swing like a pendulum. Observe what happens as it collides with the marble next to it. What happens to the marbles in the middle? What happens to the one on the opposite end? How long does the system keep moving? Record your observations. Then try modifying your design so the system will move for a longer time after you give it the initial swing.

Beyond the BookWith the help of an adult, use the Internet to research perpetual motion experiments by famous inventors and scientists, such as Leonardo da Vinci and Robert Fludd.

FOCUS Book

PerPetual Motion: Fact or Fiction?

Have you ever used a gyroscope? It is like a top. You pull a string, and then it spins for a very long time. Is it possible to make a gyroscope spin so fast that it never stops? If so, it would be in perpetual (per-PEH-chew-uhl) motion. A perpetual motion machine is a device that moves forever, without adding energy. But can such a machine exist?

The blades of a wind turbine may seem to be in constant motion, but the turbine is not a perpetual motion machine. Wind is made of air particles that have kinetic energy, the energy of motion. When the moving air blows against the turbine blades, some of the energy moves the blades. If the wind stopped, so would the turbine.

A Machine That Never Stops

The gyroscope can spin for a long time, but it will eventually slow and fall off the pencil. The wind turbines only move when air around them is moving.

FOCUS QuestionIs perpetual motion possible?

Stability and Change

PerPetual Motion: Fact or Fiction?

Perpetual Motion: Fact or Fiction? © Learning A–Z

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Photo Credits: Front cover, page 2 (right): © Nicholas Eveleigh/Alamy; page 2 (left): © David Steele/123RF; page 3 (main): © Pavle Marjanovic/123RF; page 3 (inset): © Vaclav Volrab/123RF; page 4: © REUTERS/Steve Nesius; page 5: © zentilia/123RF; page 6: © slasny/123RF; page 8: © Worldspec/NASA/Alamy; page 9: courtesy of NASA/GSFC/Reto Stöckli, Nazmi El Saleous, and Marit Jentoft-Nilsen

Illustration Credits:page 7: © Q2A Media Inc.

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Why would we want a perpetual motion machine? We need large amounts of energy to run our phones, cars, and other machines. To produce energy, we burn fossil fuels such as coal, oil, and natural gas. We also use wind farms, dams, and nuclear power plants.

These energy sources often create pollution or disrupt environments and ecosystems.

If we could invent a perpetual motion machine, it would keep moving without adding energy. In fact, its motion could be used to produce more energy. Just imagine . . . free, clean energy!

Unfortunately, no one has ever successfully designed a perpetual motion machine. Newton’s first law of motion says that an object in motion will stay in motion unless it is acted on by outside forces. This seems to mean that perpetual motion should be possible. In the real world, however, outside forces always interfere.

When a baseball is hit into the air, the forces of air resistance and gravity act on it. Air resistance is a kind of friction that slows objects moving through air. Gravity pulls the ball down to the ground.

Nuclear power (top left) and hydroelectricity (bottom) are two of the energy sources we use to run machines.

air resistance

motion of ball

gravity

An Ob ect in Motion . . .

Force and Motion • Perpetual Motion: Fact or Fiction? 3 4

Gravity is the force that attracts objects toward each other and toward Earth’s center. Because this force is always present on Earth, people have tried to make perpetual motion machines using gravity.

In the machine on this page, gravity pulls the lifted ball downward. It collides with the other balls, and the last ball flies upward, until it is pulled down again by gravity. Does this machine convert gravity into perpetual motion? It may seem like it, but no. As kinetic energy travels from one ball to the next, friction slows the movement slightly. The energy from the initial human pull to get the machine moving will eventually run out because of friction.

Friction is a force caused by two objects rubbing together. Friction changes kinetic (moving) energy to thermal energy, or heat. Think about rubbing your hands together very quickly. As you use kinetic energy to move your hands against each other, they get warm. The warmth is thermal energy caused by friction.

In every machine, friction eventually “steals” enough kinetic energy to make the machine stop. Friction changes some of the energy of motion into heat energy. Then the heat energy is transferred into the air or nearby objects. If the design is very good, there will be very little friction, but no machine runs forever without needing more energy.

Gravity Machines

Frictional Forces

a Newton's cradle

If you rub your hands together very rapidly, they soon become very warm. Friction changes some of the energy of motion into thermal energy.

Force and Motion • Perpetual Motion: Fact or Fiction? 5 6

Perpetual Motion in Space?We cannot build a perpetual motion machine on Earth, so why not try it in space? Think about satellites, such as those that monitor weather or carry phone calls. Once in space, they orbit the planet on their own.

Even in space, machines cannot avoid friction. There is very little air up where satellites orbit Earth. But there is enough to cause some air resistance. Solar winds, flowing particles from the Sun, also act as a force that pushes on objects in space.

A satellite gradually slows down over time. Most satellites have small rockets to give them an occasional boost.

Have you seen a toy drinking bird like this one? Some people call it a perpetual motion machine. But is it really?

A Very Thirsty Bird

When the head is down, material on the outside of the bird’s head absorbs water.

A different liquid inside the head flows down to the bulb. The weight of liquid in the bulb causes the bird to stand up.

As water evaporates from the material on the head, the head cools. This reduces pressure in the head. Higher pressure in the bulb pushes liquid into the head.

The head is heavier than the bulb again. The head falls, and the bird is back in the drinking position. The pattern repeats.

The drinking bird toy will not keep working unless water and heat are added. Heat from the surrounding air makes the water keep evaporating.

If a spaceship were launched into space, far from any planets and stars, could it experience perpetual motion?

Force and Motion • Perpetual Motion: Fact or Fiction? 7 8

Our planet has been rotating on its axis, similar to a gyroscope, for billions of years. The Moon orbits Earth once a month. Could this system be called a perpetual motion machine?

No. Earth’s rotation was originally much faster than it is today. The Moon was once much farther from Earth. As the Moon got closer, the force of its gravity pulled on Earth, slowing Earth’s rotation. Earth also pulls on the Moon, slowing the Moon’s orbit. Even this system loses energy over billions of years.

Whether in space or here on Earth, there has never yet been a working perpetual motion machine, but people perpetually try to design one!

Write your answers on separate paper. Use details from the text as evidence.

1 A synonym is a word that has the same meaning as another word. What are two synonyms for perpetual?

2 Choose a machine from this book. Explain why it is not an example of a perpetual motion machine.

3 Newton’s first law of motion says that an object in motion will stay in motion unless acted on by outside forces. What are three examples of outside forces?

4 Look at the photo on page 8. The satellite must use its jets to add energy, or it will begin to fall. Explain how two forces keep the satellite from being a perpetual motion machine.

5 Explain why a person who stops pedaling a bicycle on flat ground will gradually come to a stop without using brakes.

Is perpetual motion possible? Write a paragraph explaining what prevents a true perpetual motion machine from being designed, or write about how you think such a machine could be designed.

FOCUS Question

The pull of gravity between Earth and the Moon has provided energy to keep the Moon in orbit for billions of years.

Force and Motion • Perpetual Motion: Fact or Fiction? 9 10