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^
This

Is

v\

a Let's-Read-and-Firid-Out Science Book®

(^nmu^ ^ ^u:0jmi^

By

Franklyn M. Bran

^y

Illustrated

by

DOD

A/.Q'''

'sn

Why

does a ball come

up

down when

Why can

you throw

it

you

a chair without floating

sit in

in the air?

upward? And why can astronauts
bounce around so easily on the moon,
practically weightless? Gravity

is

the answer.

Using sim. pie examples, Franklyn

M. Brarley reveals what

scientists

understand about the force of grav-

He explains why you would have
a different weight on the moon and
on each of the planets, and why anyity.

thing on Earth will always be drawn

toward the planet's center. And he

reminds us

that,

though we know

certain things about gravity, no one

knrws exactly what gravity

Don Madden's

is.

lighthearted pic-

tures fciliven this easy-to-read exploration of the m.ystery of gravity.

Thomas

Y. Crowpll

Npw

Ynrk

Presch-3

$11.89

y

'1

BOSTOISI
x^

PUBLIC

UBWkVy

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«.*.

WITHDRAWN
No tonftr th« property of tte
BoolMiPubHeLibrafy.
Salo of this RMlMrial bonoffti tho UbTMy.

m/mmmmmmi

W^.^aMTf^

This

Is

a Let's-Read-and-Find-Out Science Book®

[B

^ t^JiBuaii^

ggieston Squ
2044 Columb

Roxbury,

By

MA

.':'«li)

illustrated by Don
Franklyn M. Branley
THOMAS Y. CROWELl NEW YORK

Madden

—

Other Recent Let's-Read-and-Find-Out Science Books® You Will Enjoy
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Is

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•

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into a

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What the Moon

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to

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The BASIC Book

•

Hamburger
Is

•

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and Bytes

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for

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The Let's-Read-and-Find-Out Science Book series was originated by Dr. Franklyn M. Branley, Astronomer Emeritus and
former Chairman of The American Museum-Hayden Planetarium, and was formerly co-edited by him and Dr. Roma Gans,
Professor Emeritus of Childhood Education, Teachers College, Columbia University. For a complete catalog of Let's-Readand-Find-Out Science Books, write to Thomas Y. Crowell Junior Books, Harper & Row, Publishers, Inc., 10 East 53rd Street,

New

York,

NY

10022.

Let's-Read-and-Find-Out Science Book

Gravity Is a Mystery
Text copyright © 1986 by Franklyn M. Branley
Illustrations copyright © 1986 by Don Madden
All rights reserved. No part of this book may be
used or reproduced in any manner whatsoever without
written permission except in the case of brief quotations
embodied in critical articles and reviews. Printed in
the United States of America. For information address
Thomas Y. Crowell Junior Books, 10 East 53rd Street,
New York, N.Y. 10022. Published simultaneously in
Canada by Fitzhenry & Whiteside Limited, Toronto.

Designed by Trish
10

is

a registered trademark of Harper

& Row, Publishers, Inc.

Library of Congress Cataloging-in-Publication Data
Branley, Franklyn Mansfield, 1915-

Gravity

is

a mystery.

(Let's-read-and-find-out science book)

Summary: Explains in simple text emd illustrations
what is known about the force of gravity.
1.

Gravitation

—Juvenile literature.

Juvenile literature.
iUDon 1Q97-

^QC1 78.B66

1986

)

[1.
II.

Gravity]

Title.

III.

531'.14

|_^

ISBN

0-6904)4526-3

Pjurcell

987654321

Revised Edition
(Let's-Read-and-Find-Out book)

ISBN 0-06-445057-0

I.

2.

Gravity

Madden,

Series.

85-48247

Hfc-^'""
'9.

<S9

:"<s
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•

.

/>^

O
•/",

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uppose you could dig a hole

to the center of the earth.

Suppose you dug right past the center.

If

you dug long

enough and deep enough, you would come out

in the

Indian Ocean.

^

ii\

iK
"Tf,
f

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If

you jumped into the

hole,

Down and down you would

you would

fall

down.

go.

You would fall faster and faster toward the center of
the earth.

When you

reached the center, you would be

going so fast you could not stop. You would go right past
the center.

f#
"*:<'*' -^-' .»*

V
fx

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Then, on the other side, you would move up and away

from the center

of the earth.

You would

fall

up

for a

You would go slower and slower. Then you would
You would almost get to the Indian Ocean— but

while.
stop.

not quite.

8

/

'^

I>rvUa4i

Now
earth.
center.

you would

fall

You would go

back toward the center

faster

and

of the

faster, right past the

But you would not quite reach your starting

point.

9

Back and

forth you

would

go.

Each time you would

go a shorter distance past the center.

Back and

forth,

back and

forth.

Gravity would make you fall toward the center of the
earth.
pull

When you moved past the center, gravity would

you back again.

After a long, long time you would stop moving.

would stay

You

at the center of the earth.

Gravity pulls everything toward the center of the
earth.

11

When you run downhill,
gravity pulls you.

When you throw a ball up,
gravity pulls

it

down.

When you sit,

gravity holds you down.

When you lie down, gravity holds you to the bed. But
what
12

is

gravity?

We know gravity is everywhere, even though we
can't see it. We know it pulls on every rock, every grain
of sand. It pulls on everything.

exactly

But no one knows

what gravity is. That's why we say gravity is a

mystery.

F"

The gravity of the earth pulls everything toward the

You know this when you try to lift a
Gravity pulls it down. The more the stone

center of the earth.

heavy

stone.

weighs, the more gravity pulls on

it.

To

lift

the stone,

you must pull up harder than gravity pulls down.

Lift your little brother; then try to

The more they weigh, the harder

16

it is

lift

your father.

to lift

them.

You already know how much gravity pulls on you. Do
you weigh 60 pounds? That means the pull of the earth's
gravity on you

how much
weighs

is

gravity pulls

tells

How much you weigh tells
on you. How much a stone

60 pounds.

how much

gravity pulls on the stone.

17

The earth has

gravity,

and

so does the

moon. The

earth's gravity pulls you toward the center of the earth.

were on the moon, the moon's gravity would

pull

you toward the center of the moon. The moon has

less

If you

gravity than the earth has. This

means the moon's

gravity does not pull as hard as the earth's gravity.

Do you weigh 60 pounds?

If

you were on the moon,

you would weigh only 10 pounds. The pull of the moon's
gravity on you would be only 10 pounds.

18

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:

60

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•

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-if^

•

The earth and the moon have

gravity,

and

•

^^

so does

each of the nine planets. Jupiter, Saturn, Uranus, and

Neptune have more gravity than the earth
any

of these planets

has.

On

you would weigh more than you

weigh on the earth. On Saturn, Uranus, and Neptune
you would weigh only a

would weigh a

lot

little

more.

On

Jupiter you

more.

Do you weigh 60 pounds? On

Jupiter you would

weigh about 160 pounds.

21

Gravity on Mercury, Venus, Mars, and Pluto

than
less

it is

on Earth.

On

these planets you would weigh

than you weigh on the earth.

weigh only a

is less

little less.

Pluto you would weigh a

On Venus you would

But on Mercury, Mars, and
lot less.

Do you weigh 60 pounds? On Mercury and Mars you
would weigh only about 20 pounds. Even less on

Pluto.

m

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Suppose you weigh 60 pounds. This

would weigh on the moon, on each

24

is

about

how much you

of the planets,

and on the

Pluto

8 pounds

Moon

10 pounds

Mercury

21 pounds

Mars

23 pounds

Venus

52 pounds

sun:

Earth

60 pounds

Uranus

64 pounds

Saturn

68 pounds

Neptune

85 pounds

158 pounds

Jupiter

Sun

•>i^''

16,740 pounds
25

If

you were on Mars, gravity would pull you toward the

center of Mars.

You could stand up and sit down just as you

can on the earth.

It

would be easier to move around, though,

because you would weigh
steps because gravity on
earth.

less.

Mars

You could run
is

in

big giant

much less than it is on the

You would weigh a lot more on Jupiter than you do on the
earth. It would be harder to move around on that planet. Your
legs

would get tired from carrying you. Try running with a

hundred-pound pack on your back. That's what
like all the

it

would be

time on Jupiter.

27

Gravity is everywhere
Jupiter, Mars,

gravity

too,

and

and

all

—on the earth, on the moon, on

the other planets.

The sun has

so does every other star.

The gravity of the earth holds things on the earth.
holds

28

down rugs and

tables,

and you and me.

It

29

M"^^-^

Gravity makes a ball come down.

hard

^^^

to lift a

heavy

"^11^^

It

makes us work

stone.

^:^^4l/y^,

..Ai^ii^

We know where gravity is: it is everywhere. And we
know what gravity

does.

But no one knows exactly what gravity
Gravity

32

is

a mystery.

is.

BOSTON PUBLIC LIBRARY

3 9999 01791 044 7

GKAVITY-^IS A MYSTERY

0C178

866/ 1986

87005682-37
EG

/fT)

Boston Public Library

C^/^;

EGLESTON SQUARE

BRANCH LIBRARY

The Date Due Card in the pocket indicates the date on or before which this

book should be returned

to the Library.

Please do not remove cards from
pocket.

this

Frank! vn M. Branle^

is

the

originator of the Let's-Read-and-Find-

Out Science Book

series

and the au-

thor of over 100 popular books on

young people of
all ages. He is Astronomer Emeritus
and former Chairman of the Amer-

science subjects for

ican

Museum-Hayden Planetarium.

Dr. Branley holds degrees from

New York University, Columbia Uniand the State University of
New York at New Paltz. He and his
wife live in Sag Harbor, New York.
versity,

Don Maddenhas illustrated more
than three dozen books, including

Franklyn M. Branley's
IN

OUTER SPACE? and

IS

THERE LIFE

several olhers

for the Let's-Read-and-Find-Out Sci-

ence Book series.

Mr. Madden attended the Philadelphia

Museum

College of Art. After

graduating he joined the faculty as ?.n
instructor in experimental

drawing

and design. He is the winner of seve'

al

gold and silver medals at exhibitions
of the

Club.

Philadelphia Art Directors

He and his wife, son, and daugh-

ter live in upstate

New York.

s
Jacket art

©

1986 by

Don Muddev

A Message from Roma Gans
Professor Emeritus of Childhood Edveation, Teachers College,

Columbia University

Children want to be up-to-date. As children read or are

read to, they feel themselves stretch

in

importance. This is

a feeling that all children should have in growing up. Let's-

Read-and-Find-Out Science Books are designed

them acquire

who

is

this feeling.

They are planned

to help

for the child

eager to know. Let's-Read-and-Find-Out Science

Books present basic science information, are written with
an understanding of how children think, are brief enough
for

young children

to

cope with, and are long enough to

challenge them.

W^m
rary Binding
j^gg^^