FOCUS Book

Make a simple wrecking ball using a long sock and a tennis ball. Come up with a way to suspend your wrecking ball so that it can swing like a pendulum. Next, design a building out of blocks and draw a diagram of it. Then keep the top of the sock in one place, and use your wrecking ball to knock down the building. How many swings did it take to knock it down? Did some blocks remain standing?

Now design a wrecking ball that knocks down your building even faster, or more completely. First, draw a diagram of it, then build it. Test your idea using the same blocks and building model. Record your observations. Were you able to make a better wrecking ball?

Use the Internet to find other examples of energy transfer. Think about how they are and are not like wrecking a building.

Beyond the Book

Strong Wall

Wre

cking Ballvs.

Have you ever tried to knock over a sand castle? Wrecking a weak building made of sand is easy. What if your job were to demolish a strong building made of brick and concrete? You could use a wrecking ball!

Before knocking down an old building, workers first make sure that no one is inside. Then they bring a crane with a chain and a very heavy metal ball attached. This is a wrecking ball machine. Once the workers are sure that everyone is safe, they can swing the wrecking ball.

Demolition Day

How does a wrecking ball knock down a building?

Energy and Matter

FOCUS Question

a wrecking ball machine

Strong Wall

Wre

cking Ballvs.

Wrecking Ball vs. Strong Wall © Learning A–Z

All rights reserved.

www.sciencea-z.com

Photo Credits: Front cover, page 9: © iStock/ShooteRhys; page 2: © Photoarts30/Dreamstime.com; page 3 (top): © Golden Pixels LLC/Alamy; page 3 (bottom): © Pixelcarpenter/Dreamstime.com; page 6: © de.photoarts/Alamy; page 7 (left): © Timur Druzhynin/123RF

Illustration Credits:page 4, 5, 8: © Q2A Media Inc.

2

One way to use a wrecking ball is to drop it straight down. Think about holding a bowling ball. Suppose you could lift it over an old building and drop it straight down. It would smash through the roof! The higher the ball is lifted, the more damage it causes when it falls. This is due to gravity.

Gravity always pulls things down. When an object falls, gravity makes it fall faster and faster. An object that falls a long way moves with more energy than if it falls a short way.

Dropping the wrecking ball straight down is often used to punch through roofs or floors.

Another way to use a wrecking ball is to swing it. The ball is pulled back toward the crane. Then the ball is released. It swings forward and smashes into a wall.

The force of this forward motion is how wrecking balls knock down walls.

When a wrecking ball machine is used in this way, it acts as a pendulum.

Swinging from the side is the most common way to use a wrecking ball.

Tall Crane, Swinging ChainGravity Drop

Machines • Wrecking Ball vs. Strong Wall 3 4

A pendulum is an object that swings back and forth from a fixed point. It has a weight at the end of a line. When the pendulum is lifted and released, it swings back and forth in a curved path, or arc. At the top of its swing, gravity pulls the weight down. It speeds up as it goes down. After it passes the bottom part of its swing, the pendulum starts to move up along the arc. Gravity slows the pendulum down until it stops at the other end of the arc. Then gravity pulls it down again. It swings back in the other direction.

A wrecking ball works in a similar way. But a wall gets in the way before the ball can make one full swing through its arc.

A wrecking ball machine uses a lot of energy to demolish a building. Energy is the power to cause movement or change. When the ball is pulled back toward the crane, the machine has stored-up energy.

Whoosh! The wrecking ball swings. Thunk! The ball smashes into the wall. The building starts to crumble. A cloud of dust swirls into the air. Workers use other machines to haul away the rubble.

Energy from a swinging wrecking ball knocks down a building.

PENDULUM

stored energy turns to energy of motion

gravity pulls down on the ball ball

speeds upball slows down

energy of motion turns back into stored energy

Pendulums Wrecking Ball Energy

ball swings back again

Machines • Wrecking Ball vs. Strong Wall 5 6

A moving wrecking ball has energy as it swings. This energy is passed along, or transferred, to the wall when it hits. It breaks the wall apart.

Have you seen a toy called Newton’s cradle? The toy shows another example of energy transfer. A swinging ball crashes into a line of nonmoving balls. Energy is transferred to each of the balls in the middle, and then to the last one in line.

What happens next? Only the last ball flies up. The energy from the first ball was transferred to the last ball.

A Better Wrecking BallEnergy TransferBuildings can be made out of many different kinds of materials. Some are easy to knock down. Others are more difficult. Not to worry, though. A wrecking ball machine can be adjusted for different jobs.

One way is to use a longer chain. This means that the wrecking ball can be lifted higher. A higher ball swings and hits the wall with more energy. A wrecking ball with more energy can knock down strong walls.

A Newton's cradle is an example of how energy can transfer from one object to another.

A wrecking ball on a shorter chain (above) does not have as much energy as a wrecking ball on a longer chain (right).

What would happen if the wrecking ball were not pulled all the way back to the crane?

Machines • Wrecking Ball vs. Strong Wall 7 8

How does a wrecking ball knock down a building? Explain the features of the best wrecking ball machine for a big job. Describe how it uses energy to do its job.

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

1 Describe two ways to use a wrecking ball.

2 Look at the diagram on page 5. Where does the ball have the most energy of motion? Why?

3 Compare and contrast a Newton’s cradle with a wrecking ball machine.

4 Describe an example of energy transfer that you have observed. What purpose did the energy transfer serve?

5 Choose the best word to complete this sentence correctly: The higher a wrecking ball is pulled back, the

it will swing when the rope lets it go.

Workers can also use a heavier or lighter wrecking ball. A heavier ball moves with more energy than a lighter one. It has more mass. A large wrecking ball can weigh about the same as a pickup truck. A lighter wrecking ball is easier to lift higher.

The first wrecking balls were round. Now most are shaped more like pears. When a wrecking ball is narrower at the top, it is easier to pull out of holes in buildings.

FOCUS Question

For certain buildings, a wrecking ball machine just is not strong enough. In this case, workers might use explosives to demolish it. Boom! The explosion weakens the walls. Gravity causes the building to collapse on itself.

Machines • Wrecking Ball vs. Strong Wall 9 10