What Holds Us to Earth?: A Look at Gravity (Lightning Bolt Books -- Exploring Physical Science)

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Jennifer Boothroyd

s

Lerner Publications Company Minneapolis

To m y d n e i r f l a i c e p s E m il y — et r fe

he s p d e n e a k d o w h e g ro u n on t h a d in t he e h r e h s d u o cl Copyright

© 2011 by Lerner Publishing Group, Inc.

All rights reserved. International copyright secured. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means— electronic, mechanical, photocopying, recording, or otherwise— without the prior written permission of Lerner Publishing Group, Inc., except for the inclusion of brief quotations in an acknowledged review. Lerner Publications Company A division of Lerner Publishing Group, Inc. 241 First Avenue North Minneapolis, MN 55401 U.S.A. Website address: www.lernerbooks.com

Library of Congress Cataloging-in-Publication Data Boothroyd, Jennifer, 1972– What holds us to earth? : a look at gravity / by Jennifer Boothroyd. p. cm. — (Lightning bolt booksTM —Exploring Physical Science) Includes index. ISBN 978–0–7613–5430–7 (lib. bdg. : alk. paper)  1. Gravity—Juvenile literature. I. Title. QC178.B634 2011 531’.14—dc22 2009038499 Manufactured in the United States of America 1 — BP — 7/15/10 eISBN: 978-0-7613-6299-9

Contents Something Is Different A Strong Pull

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4

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6

Center of Gravity Gravity In Space What Is Weight?

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13

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15

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20

Floating through Life

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24

Further Activity Glossary Reading page

Index

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28

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Something Is Different Look at this picture. Does something look out of place?

You are right! This woman

is floating in the air. 4

How is that possible? The woman is an astronaut in outer space. People float in space because there’s not much gravity there.

Outer space has only a little bit of gravity. Astronauts call it microgravity. Micro means “very little.”

5

A Strong Pull Gravity is a force. It pulls things toward the center of Earth. A force is a push or a pull. A ball falls to the ground when you drop it because of gravity’s pull.

6

You cannot see gravity. But you can see how it affects things. It pulls on waterfalls. It pulls on falling leaves. It pulls a boy down a slide.

7

Watch out! A plate will fall to the floor if you drop it. Gravity makes objects fall until something stops them.

8

The hard floor will stop a dropped plate from falling. And gravity will keep the plate on the floor.

Without gravity, the pieces of this broken plate would float away!

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Hey, batter, batter! Swing! This batter hits the ball hard.

10

Th str on

b a t r i e s h f o e c a v r g ity . r n a o h f er t e g

The ball soars high in the air. A thrown baseball would fall to the ground if the force from a bat didn’t make it soar.

11

But wait! The force from the bat is weakening. Gravity is pulling the ball down. A player waits to catch it.

12

When a player catches a ball, she stops gravity from pulling the ball down to Earth.

Center of Gravity Gravity helps things balance. Everything has a center of gravity. A center of gravity is a balancing point. This model seesaw’s center of gravity is at the center of the board. The seesaw can balance if it’s supported at its center.

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Gravity pulls equally on an object’s center of gravity.

That’s why objects can stay balanced This pencil can stay if they’re balanced because it’s supported at its supported at center of gravity. this point .

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Gravity In Space Gravity’s pull helps us live on Earth. It keeps water from floating away. It lets things stay in place.

Many people’s drinking water comes from lakes and streams. If we didn’t have gravity, this water would float away!

15

ro Ot

.

he

i ty

ace also ha p s ve in s t g c ra e j v b

Objects with more mass have more gravity.

Mass is the amount of matter in an objec t . 16

The sun has a lot of mass. The sun’s gravity is very strong.

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has little n o o ma m e ss. Th

Its 18

. k g rav a e w i t y is

Astronauts bounce and float when they walk on the moon!

Astronaut Edgar Mitchell walked on the moon in 1971.

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What Is Weight? How much do you weigh?

Your weight is how much gravity pulls on your body.

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This scooter weighs about 180 pounds (82 kilograms). On Earth, you weigh less than this scooter. Gravity pulls on the scooter more.

21

Imagine you visit the moon with the scooter. Both you and the scooter have the same mass as you did on Earth. You didn’t lose any parts!

22

But on the moon, the scooter weighs about 30 pounds (14 kg). You weigh even less. Since there’s less gravity on the moon than on Earth, things weigh less on the moon.

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Floating through Life Do you remember the astronaut at the beginning of this book? Astronauts need to learn how to live with little gravity.

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They need to learn to hold onto things in space, or the things will float away.

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As

e e d n t n o lea e v e rn s t u a n o tr how to work u psi de do wn

!

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Take a look around you. Gravity is everywhere.

Can you imagine your life without it?

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Activity

Galileo’s Drop Galileo Galilei was a scientist in the 1600s. He discovered that gravity makes objects fall at the same speed no matter their weight. Follow these steps to try an experiment like Galileo’s. What you need: a cookie sheet a table a baseball a tennis ball

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W hat you do : 1. Place the cookie sheet on the floor at the edge of the table. 2. Take the baseball in one hand and the tennis ball in the other. Then hold the balls over the cookie sheet while resting your wrists on the table’s edge. The table will help you keep your hands steady and even. That way, you’ll be sure that you’re dropping the balls from the same height. 3. Drop the balls at the same time. When they land, you’ll hear them hit the cookie sheet. The noise will help you know for sure which ball landed first. So which ball landed first? Or was it a tie? Were the results of your experiment the same as Galileo’s?

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Glossary astronaut: someone who travels in space

center of gravity : a point where an object balances

force: a push or a pull

gravity: a force that pulls things down toward the surface of Earth

mass: the amount of matter in an object

matter: what things are made of

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Further Reading Boothroyd, Jennifer. Give It a Push! Give It a Pull!: A Look at Forces. Minneapolis: Lerner Publications Company, 2011. Branley, Franklyn M. Gravity Is a Mystery. New York: Collins, 2007. Dragonfly TV: Microgravity http://pbskids.org/dragonflytv/show/microgravity.html Hopwood, James. Cool Gravity Activities: Fun Science Projects about Balance. Edina, MN: Abdo, 2008. Science of Baseball: The Scientific Slugger http://exploratorium.edu/ baseball/scientificslugger .html Your Weight on Other Worlds http://www .exploratorium.edu/ ronh/weight/ index.html

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Index activity, 28–29 astronauts, 5, 19, 24–26 center of gravity, 13–14 Earth, 6, 15, 21–23 floating, 4–5, 15, 19, 25 force, 6, 11–12

Galileo Galilei, 28–29 mass, 16–18, 22 moon, 18–19, 22–23 space, 5, 16, 25 sun, 17 weight, 20–21, 23, 28

Photo Acknowledgments The images in this book are used with the permission of: © Alan Bailey/Getty Images, p. 2; NASA, pp. 4, 24, 25; © Barry Blackman/SuperStock, p. 5; © Bellurget Jean Louis/ Stock Image/Getty Images, p. 6; © iStockphoto.com/mammamaart, p.6 (inset); © DK Stock/David Deas/Getty Images, p. 7; © Shannon Fagan/The Image Bank/Getty Images, p. 8; © Flirt/SuperStock, p. 9; © Flip Chalfant/The Image Bank/Getty Images, p. 10; © Rob Atkins/Photographer's Choice/Getty Images, p. 11; © John Burcham/National Geographic/Getty Images, p. 12; © Westend61/SuperStock, p. 13; © Brand New Images/ Stone/Getty Images, p. 14; © iStockphoto.com/Robert Dupuis, p. 15; NASA and The Hubble Heritage Team (STScI/AURA), p. 16; SOHO - EIT Consortium, ESA, NASA, p. 17; © iStockphoto.com/Rafael Pacheco, p. 18; NASA/JSC, p. 19; © image100/Alamy, p. 20; © iStockphoto.com/Hedda Gjerpen, pp. 21, 23 (inset right); © Prisma/SuperStock, p. 22; NASA/MSFC, pp. 23 (background), 26; © Dave King/Getty Images, p. 23 (inset left); © Chris Windsor/Taxi/Getty Images, p. 27; © Lew Robertson/StockFood Creative/Getty Images, p. 28 (cookie sheet); © Sam Lund/Independent Picture Service, p. 28 (baseball); © iStockphoto.com/Daniel Cooper, p. 28 (tennis ball); © iStockphoto.com/Don Nichols, p. 28 (table); © Photodisc/Getty Images, p. 30; © Chuck Franklin/Alamy, p. 31. Front cover: © David Ball/Alamy

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A ball drops to the ground. Leaves fall from a tree. Gravity is at work all around you. But what exactly is gravity? And how does it affect different objects? Read this book to find out!

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Learn all about matter, energy, and forces in the Exploring Physical Science series—part of the Lightning Bolt BooksTM collection. With high-energy designs, exciting photos, and fun text, Lightning Bolt BooksTM bring nonfiction topics to life!

Exploring Physical Science Attract and Repel: A Look at Magnets Give It a Push! Give It a Pull!: A Look at Forces Many Ways to Move: A Look at Motion What Floats? What Sinks?: A Look at Density What Holds Us to Earth?: A Look at Gravity Why Do Moving Objects Slow Down?: A Look at Friction