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by LISA BENJAMIN
Table of Contents
Chapter
IT]
Forces and Motion
Do you like to play baseball or basketball?
Do you like to ride a bike or swim? These
sports may sound like fun. But they are
more than just fun. Taking part in sports is
also work.
In science, work is anything that needs
force. A force is a push or a pull that
makes something move. Force helps you
hit a baseball. Force helps you pedal a
bicycle. Sports just would not be sports
without forces .
II
0
Forces on the Field
Chapter
Picture yourself at an exciting baseball
game. A pitcher uses force to throw the
ball. He pushes it with his hand. That sets
the ball in motion. It will go for if the
pitcher uses a lot of force.
The ball flies toward home plate. The
batter hits the ball hard. That, too, is
an example of force. The batter pushes
the ball with her bat. The ball changes
direction. It flies out into right or left field.
Forces can also stop things from moving.
Picture the ball flying toward a player
on the field. The player reaches up and
catches the ball. He pulls the ball from the
air and stops its motion.
But what happens if the player does not
catch the ball? It falls to the ground. That
happens because of a force called gravity.
Gravity pulls things toward the center of
Earth. It keeps us on the ground. Without
gravity, everyone and everything would
float upward.
Chapter
@]
Forces on the Road
,/
Forces help in other sports, too. You use
force anytime you ride a bicycle. You push
the pedals to make the bicycle move. The
harder you push, the faster you go.
It is easy to ride a bicycle on a smooth
road. But it is not so easy to ride on a
bumpy road . That takes a lot of force. You
have to push the pedals much harder to
make the bike move.
The bumps in the road cause friction.
Friction is another kind of force. It slows
things down. It makes it harder to move.
Chapter
[i]
Forces in the Water
Swimmers use force to pull their bodies
through the water. You also need force
to row a boat. You push oars through the
water to move the boat.
The water can be a force, too. It can
push against the boat and change the
boat's direction . Water also causes
friction . It is hard to row a boat through
choppy water.
m
Chapter ~
Forces, Tools, and Machines
It takes a lot of force to build things,
too. That is why people use tools. Tools
give us more force to make things move.
A hammer is a kind of tool. It helps us
build things.
We often use a hammer to nail wood
together. It pushes the nails hard into
the wood . Hammers also help us pull
things apart.
Machines help us do work, too. They
help us move big heavy things. Workers
use a bulldozer to push away dirt.
Forces help us build basketball courts.
They help us shoot balls through
hoops. There is not much we could do
without forces.
force (FAWRS) a push or pull that
makes something move or change
direction (page 3)
friction (FRIK-shuhn) a force that slows
down moving things (page 9)
gravity (GRAV-i-tee) a force that pulls
things toward Earth (page 7)
machine (muh-SHEEN) a tool, usually
with moving parts, that
makes work easier to do (p age 14)
motion (MOH-shuhn) a change in
position (page 4)
tool (TEWL) something you hold in
your hand to do work (page 12)
work (WURK) the force that changes
the motion of an object (page 2)
boat, 10-11
machine, 12, 14
bulldozer, 14
motion, 2, 4, 6
direction, 5, II
pull, 3, 6-7, 10, 13
Earth,7
push, 3-5, 8-11,
force, 2-14
13-14
friction, 9, II
tool,12-13
gravity, 7
work, 2-3, 14
hammer, 12-13
worker, 14
Activities
Directions Wr ite your answers to these
questions in yo ur science journal. Then
comp lete the reading and wr iting act ivit ies.
Responding to Science
1. What is your favorite game or sport? Does
it t ake more pushing or more p ulling?
(Reader's Response)
2. Why is it hard to row a boat through rough
water? (I nterpret)
3. Have you ever played tug of war? What
kind of force did you use? (Apply)
Reading in Science
00
Cause and Effect W ith a partner,
ta lk about what causes friction
Cal'''
and gravity. What happens as a
result? Fill in a cause-and-effect chart.
Effect
Writing in Science
Sports Article Suppose you were a sports
writer at a baseba ll game. Explain how forces
affect t he game you are watching.
Writing That Explains