3 12 forces and motion (physical science)

Physical Science

by Ann J. Jacobs

Genre

Nonfiction

Comprehension Skill

Summarize

Text Features

• Captions
• Chart
• Glossary

Science Content

Forces in Motion

Scott Foresman Science 3.12

ISBN 0-328-13841-X

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Vocabulary

What did you learn?

force

1. What is the difference between constant speed
Ann J. Jacobs
and variable by
speed?

friction
gravity
magnetism
motion

Forces and Motion

2. Would it take more force to begin moving
an empty wagon or a wagon loaded with
pumpkins? Why?

position

3. What is the difference between a contact force
and a non-contact force? Give examples of each.

relative position

4.

In this book you have

read about work and simple machines. Write to
explain how simple machines make work easier
without lessening the amount of work that gets
done. Use examples from the book.

5.

Summarize Turn to p. 11. Summarize the
forces used when riding a bike.

speed
work

Photographs: Every effort has been made to secure permission and provide appropriate credit for
photographic material. The publisher deeply regrets any omission and pledges to correct errors called to its
attention in subsequent editions. Unless otherwise acknowledged, all photographs are the property of Scott
Foresman, a division of Pearson Education. Photo locators denoted as follows: Top (T), Center (C), Bottom
(B), Left (L), Right (R) Background (Bkgd)
Opener: ©Allsport Photography/Getty Images; 2 ©David Young-Wolff/PhotoEdit; 4 ©Roy Ooms/
Masterfile Corporation; 5 ©John Elk/Bruce Coleman Inc.; 6 (TL) ©Richard Megna/Fundamental
Photographs, (CR) ©Stone/Getty Images, (BL) ©Tom Szuba/Masterfile Corporation; 7 ©Kunst &
Scheidulin/Age Fotostock; 8 ©Jim Cummins/Corbis; 10 ©Rolf Bruderer/Corbis; 11 ©Stone/Getty
Images; 12 ©Hennie Wiggers/Zefa/Masterfile Corporation; 14 ©Taxi/Getty Images; 15 AlaskaStock;
16 ©Bob Daemmrich/Corbis; 17 ©DK Images; 18 (CL) Getty Images, (R) ©Tony Freeman/PhotoEdit; 20
Corbis; 22 ©Keith Pritchard/Alamy Images; 23 ©Neil Rabinowitz/Corbis

ISBN: 0-328-13841-X
Copyright © Pearson Education, Inc.
All Rights Reserved. Printed in the United States of America. This publication is
protected by Copyright and permission should be obtained from the publisher prior

to any prohibited reproduction, storage in a retrieval system, or transmission in any
form by any means, electronic, mechanical, photocopying, recording, or likewise. For
information regarding permissions, write to: Permissions Department, Scott Foresman,
1900 East Lake Avenue, Glenview, Illinois 60025.
3 4 5 6 7 8 9 10 V010 13 12 11 10 09 08 07 06 05


What happens when
things change position?
When Things Move
Suppose you spin a top. You can tell it moves
because its location changes. It starts in your hand.
Then it spins across the ground. If an object is in a
new location, its position has changed.
The top is in motion as its position changes. A
top also has circular motion. This means it moves in
circles around a central point.

Ways of Looking at an Object’s Position
The position, direction, and movement of an object
depend on how you look at it. For example, as you
walk down a hall, a water fountain may be in front of
you. After you walk by it, it is behind you. As you keep
walking, the fountain seems to be moving away.
A map can help you locate things. A map shows
the position of objects in relation to each other. Look
at the path on the map above. Tell how you would get
from the classroom to the lunch room. Use position
words like forward, left, right, and behind to explain
your trip.

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3


Positions of Moving Objects
When one thing depends on another thing

Look at the train below. The locomotive is in front.
Two cars are being pulled behind it. Could the cars

for its meaning, we say it is relative. Relative

move in front of the locomotive? They could if the

position is the position of one object compared

train started moving backward. Relative position can

with the position of other objects. Look at car 64

change if objects start moving in a new direction.

below. Its relative position is in front of the other
cars. Another car might be leading the
race. Then you would say car 64 is behind
the lead car. The relative position of
car 64 changes depending on the
position of other cars.

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5


How Fast Things Move

Constant Speed

Speed is how fast an object changes its position.
Speed can be fast. A jet plane moves fast. Speed can
be slow. Honey moves slowly. Some things have such a
slow speed that you can’t see them move!
The arms of a tuning
fork move very fast.

Objects can move at a constant speed. This means
that they do not change how fast or slow they move.
They are always changing position. The rate at
which they change position stays the same.

Variable Speed
Bumper cars move at a variable speed. An object
moving at a variable speed changes speed as it moves.
Bumper cars can speed up, slow down, or stop.

Honey moves at
a slow speed.

The motion of

these flowers is
too slow to see.

6

7


How does force
affect motion?
The Causes of Motion
A force is any push or

The mass of an object affects how it moves. You
don’t need much force to begin pushing an empty
cart. When the cart is full of food, it has more mass.
Now more force is needed to begin moving the cart.
As the cart moves, its wheels rub against the floor.
This causes friction. Friction is a contact force that

pull. A force can change

slows an object’s motion. Smooth surfaces produce less

an object’s position. It can

friction than rough ones. Friction is helpful when you

change the direction of an

want to slow something down.

object’s motion too.
Most forces you use are
contact forces. To push or
pull an object, you must
touch it. A bat makes contact
with a ball to move it.

More force is needed
to push three carts
than to push just one.

The more force you use,
the more an object’s motion
changes. The harder you
push on a cart, the faster
it goes!

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9


Motion and Combined Forces
You know that pushes, pulls, and friction can

Lots of forces cause a bike to change its motion.
You push on the pedals. You push on the handlebars

change an object’s motion. Now think about the

and shift your weight to turn. Friction between the

forces in tug-of-war. Your team’s pull is a force in

tires and the ground slows your forward motion.

one direction. The other team’s pull is a force in the

Going up hills takes more force. You may gain too

other direction. If the forces are the same, the rope

much speed going down hills. You pull on the brakes

doesn’t move.

to slow down. All these forces work together to keep

Suppose everyone on your team pulls together to

your bike moving.

move the rope. The rope will move in your direction if
you have the stronger force. So get more friends to help!

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11

Gravity and Magnetism
A non-contact force is a push or pull that affects an
object without touching it. Gravity is a non-contact
force that pulls objects toward each other. Gravity
pulls everything on Earth toward Earth’s center.
Weight is the amount that gravity pulls on an

The pull of gravity also depends on matter. Objects
with more matter have more mass. The more mass,
the stronger the pull of gravity.
Magnetism is another non-contact force. Magnets
pull on objects with certain kinds of metal such as iron
in them. You might use a magnet to pull a steel paper

object. Objects weigh less on the Moon because the

clip across your desk. Steel is a metal with iron in it.

Moon has less gravity than Earth. The pull of gravity

Magnets do not affect objects that lack these metals.

is less the farther you are from the center of Earth.
You weigh less on top of a mountain than you do
at it’s base.
This magnet does not attract
crayons since they lack metal
the magnet can attract.

This magnet attracts
paper clips because
they have iron.

Gravity pulls skydivers
toward the ground.

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13


How do simple
machines affect work?
Work
In science, work means using force that moves an
object. You do work when you rake leaves or kick a
ball. How much work you do depends on how much
force you use. It also depends on how far you move an
object.
Work is NOT done if an object does not move. Try
pushing a big snowball. If the snowball does not
move, no work is done. You may be using force, but
you are not doing work.

When Work Is Done
Activity

14

Work

How much work do you do in a day? To find out,
you would have to keep track of all the pushing and

Thinking about a
math problem

No

pulling you do. Then you would have to see how far

Turning a jump rope

Yes

those pushes and pulls moved things.

Holding a puppy

No

Lifting a puppy

Yes

Pulling on a
locked door

No

Opening an
unlocked door

Yes

Trying to scoop
rock-hard ice cream

No

Putting a book on a shelf is work. What if you put
the book on a shelf twice as high? You would do twice
as much work! You would also do twice as much work
if you put a book that weighed twice as much on the
lower shelf.
15


Some Simple Machines
Machines can’t reduce the amount of work that

Wedge
A wedge is a simple machine. It is used to split,

needs to be done. But they can help make your work

cut, or fasten things. A wedge has one or two

easier. Here are six kinds of simple machines.

slanted sides that form a sharp edge. A

Inclined Plane

wedge can move through material and

Look at the man pushing the cart below. Without
the ramp, the man would have to lift the cart into the
truck. That would take a lot of effort. He is using a

separate it. A knife is a wedge that
can cut a pie. An axe head is a
wedge that can split wood.

machine called an inclined plane. An inclined plane,
or ramp, is a slanting surface. It connects a low spot to
a higher one. The mover pushes with less force, but he
must push for a longer distance. The same amount of
work gets done, but with less effort.

16

17


Screw
A screw is an inclined plane wrapped around a

Support

center post. The spiral slide in the picture looks like a
big screw.
Screws are used to hold
things together. They are
used to raise and lower
things too. The lid on
a jar is a screw. The
lid raises as you
open the jar. The
lid lowers as you
close the jar.

Lever
A lever is another simple machine. A lever is a bar
that rests on a support. Levers lift and move things.
A seesaw is a lever. Push down on one side of a seesaw.
Then watch the person on the other side go up!

18

19


More Simple Machines
Wheel and Axle
Whenever you open a door, you use a simple
machine. It is called a wheel and axle. The doorknob
is a wheel. The post that attaches to its center is an

axle. Turning the knob takes less force than turning

The rides you see below both use a big wheel
and axle. Instead of turning the wheel, a motor in
these rides turns the axle. The motor turns the axle
a small distance. But the distance the axle turns the
wheel is great. This makes the ride fun. The motor
must put a great force on the axle to do this.

the small axle with your fingers. Since the knob is
larger, however, you must turn it a longer distance.

Do you think the people on
this ride are having fun?

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21


Pulley

You live in a world of motion. Look around. What
is moving quickly? What is moving slowly? What

Pulleys are simple machines that can help move
things in the right direction. The sails of a boat are

simple machines will you use today to make moving
things easier?

attached to pulleys. A pulley has a wheel that turns on
an axle. The sailors pull on a rope that wraps over the
wheel of each pulley. This causes the sail to move in
the proper direction.

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23


Glossary
Vocabulary

What did you learn?

force
force

any push or pull

friction
friction

a force that goes against the

gravity

motion of an object

gravity

a non-contact force that pulls

magnetism
motion

objects toward each other

magnetism

a non-contact force that pulls on

position

certain kinds of metal such as

relative position

iron

motion
speed

a change in the position of

work

the location of an object

relative position

the position of one object
compared with the position of

Photographs: Every effort has been made to secure permission and provide appropriate credit for
photographic material. The publisher deeply regrets any omission and pledges to correct errors called to its
attention in subsequent editions. Unless otherwise acknowledged, all photographs are the property of Scott
Foresman, a division of Pearson Education. Photo locators denoted as follows: Top (T), Center (C), Bottom
(B), Left (L), Right (R) Background (Bkgd)
Opener: ©Allsport Photography/Getty Images; 2 ©David Young-Wolff/PhotoEdit; 4 ©Roy Ooms/
Masterfile Corporation; 5 ©John Elk/Bruce Coleman Inc.; 6 (TL) ©Richard Megna/Fundamental
Photographs, (CR) ©Stone/Getty Images, (BL) ©Tom Szuba/Masterfile Corporation; 7 ©Kunst &
Scheidulin/Age Fotostock; 8 ©Jim Cummins/Corbis; 10 ©Rolf Bruderer/Corbis; 11 ©Stone/Getty
Images; 12 ©Hennie Wiggers/Zefa/Masterfile Corporation; 14 ©Taxi/Getty Images; 15 AlaskaStock;
16 ©Bob Daemmrich/Corbis; 17 ©DK Images; 18 (CL) Getty Images, (R) ©Tony Freeman/PhotoEdit; 20
Corbis; 22 ©Keith Pritchard/Alamy Images; 23 ©Neil Rabinowitz/Corbis

other objects

how fast an object changes
its position

work

ISBN: 0-328-13841-X

when an object is moved by
a force

Copyright © Pearson Education, Inc.

All Rights Reserved. Printed in the United States of America. This publication is
protected by Copyright and permission should be obtained from the publisher prior
to any prohibited reproduction, storage in a retrieval system, or transmission in any
form by any means, electronic, mechanical, photocopying, recording, or likewise. For
information regarding permissions, write to: Permissions Department, Scott Foresman,
1900 East Lake Avenue, Glenview, Illinois 60025.
3 4 5 6 7 8 9 10 V010 13 12 11 10 09 08 07 06 05

24

2. Would it take more force to begin moving
an empty wagon or a wagon loaded with
pumpkins? Why?
3. What is the difference between a contact force
and a non-contact force? Give examples of each.
4.

In this book you have
read about work and simple machines. Write to
explain how simple machines make work easier
without lessening the amount of work that gets
done. Use examples from the book.

5.

Summarize Turn to p. 11. Summarize the
forces used when riding a bike.

an object

position

speed

1. What is the difference between constant speed
and variable speed?