FOCUS Book
A pinwheel spins in the breeze. Design and build
a simple pinwheel with paper blades that will spin
fast. To make it spin well, you will need to reduce
the friction between the pinwheel and its base.
Design your pinwheel with parts or materials that
create very little friction. What are some other
ways your design can reduce friction and help the
blades spin faster? Test your pinwheel in front of
a fan. Keep the fan at a steady speed and keep
your pinwheel at the same distance for every test.
Talk to other students about their pinwheels.
Observe which design spins fastest. What
changes can you make to improve your design?
Beyond the Book
Friction with the air is called wind
resistance. Go online to explore how
athletes in different sports reduce
wind resistance.
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Warm Up with Friction
Science
Friction
It’s a cold day, and you forgot your mittens. You
rub your chilly hands together. Suddenly they
feel warmer. Where did that warmth come from?
The warmth in your hands came from heat energy
generated by friction. Friction is a force. It acts
between objects that rub against each other.
Friction also causes moving things to slow down.
It changes the energy of a moving object from
motion into heat.
FOCUS Question
How is friction related to heat
energy?
Cause and Effect
Photo Credits:
Front cover, page 9 (right): courtesy of NASA; page 2: © STOCK4B GmbH/Alamy; page 3: © Jacek Chabraszewski/
123RF; page 4 (left, top to bottom): (1) © khorzhevska/iStock/Thinkstock; (2) © Grassetto/iStock/Thinkstock;
(3) © Joggie Botma/iStock/Thinkstock; (4) © ViktorCap/iStock/Thinkstock; page 4 (right, top to bottom):
(1) © 1971yes/iStock/Thinkstock; (2) © Grassetto/iStock/Thinkstock; (3) © marcutti/iStock/Thinkstock; (4) © Aksonov/
iStock/Thinkstock; page 5 (top): © Milan Vasicek/Hemera/Thinkstock; page 5 (bottom): © veryan dale/Alamy;
page 6 (top): © dmitrydesign/iStock/Thinkstock; page 6 (bottom): © Gregory21/Dreamstime.com; page 7:
© Lawrence Weslowski Jr/Dreamstime.com; page 8 (top): © iStock/dandanian; page 8 (bottom): courtesy
of NASA/JPL/University of Arizona; page 9 (left): © Alan Dyer, Inc/Visuals Unlimited/Corbis
Science Friction
© Learning A–Z
Written by Katherine Follett
Friction can help you warm your hands on a cold day.
All rights reserved.
www.sciencea-z.com
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Factors of Friction
What Is Friction?
It’s harder to walk through water than through air. If
you run through water, it only gets harder. Water has
more particles in the same amount of space than air
does. More particles mean more friction.
You use friction every day. When you walk, run,
jump, and stop, your feet touch the ground and
create friction. If you wear skates with wheels,
you have less friction with the ground. Zoom!
As speed increases, so does friction. Mass, or the amount
of matter, creates more friction. It is easier to push an
empty box than a full one. The size of a surface can also
affect friction. It is easier to push a skinny box than a wide
box. A rough surface causes more friction. It’s easier to
push a box across a smooth floor than across a carpet.
Less
friction
How will friction be important in this race?
Friction happens between all kinds of matter—
solids, liquids, and gases. Matter makes up
everything around you. All matter is made of tiny
particles. When things rub together, the edges
of the particles catch. This is friction. Friction
happens whenever matter touches.
As particles catch and pull apart again, they move
back and forth. The movement of particles is heat.
Heat Energy • Science Friction
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more
friction
low
speed
high
speed
small
mass
large
mass
small
surface
wide
surface
smooth
surface
rough
surface
Fire from Friction
Machinery and Friction
Friction changes motion into heat. Lots of friction
makes lots of heat. If wheels are moving fast enough,
the tires will start to smoke and burn (see cover).
At times, you can get enough
heat to start a fire!
Machines can create lots of friction and, as a result,
lots of heat. When machines get too hot, their parts
can short-circuit, melt, or even catch fire.
Matches use friction to create
a flame. The head of a match
catches fire at a fairly low
temperature. The match lights
when it is rubbed quickly
across a rough surface. This
friction gives off heat, so
the match catches fire.
Many machines have ways
to prevent friction. Some
have tiny parts called ball
bearings between their
moving parts. The
bearings roll past each
other. Rolling causes a lot
less friction than sliding.
Other machines use
lubricants, or slippery
materials, to prevent
friction. Oil and grease
are lubricants.
A match head catches fire
when friction heats it up.
Some people can light a fire
by rubbing two sticks together.
Wood must be at a much
higher temperature than
Spinning a stick against
a match head before it will
another stick causes friction.
Eventually, a flame may start.
catch fire. It takes a lot
of skill—and a lot of
Never light matches without
patience—to light a
the help of an adult.
fire by using friction.
Heat Energy • Science Friction
ball bearings
Machines that spin, such as drills
and wheels, have bearings inside.
Some machines need
friction to do their jobs.
Water cools this drill bit as it cuts
through metal.
A saw can’t cut wood
without rubbing against it, but the motion heats
the blade. When machines get too hot, people
cool them using water, fans, or air conditioners.
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Friction in Cars
Internal Friction
Cars are complex machines with many moving parts.
Friction in a car is sometimes useful. Friction between
the tires and the road allows the car to move.
You have learned that friction happens when two
objects touch each other. It also happens inside an
object when it bends, stretches, or changes shape.
This is called internal friction. This type of friction
also makes heat.
Brakes also use friction. Brakes slow a car by rubbing
against the wheels. Car brakes can get very hot, so
they are made of special materials that will not crack
or melt in the extreme heat.
Some of the
When you bend an object, the
paper clip’s
object’s particles catch on each
mechanical
energy is
other. This causes friction. Try
converted to
thermal energy.
bending a metal paper clip back
and forth several times. Briefly touch the spot
where you bent the clip. It will feel warm!
For each 10 km per hour that a car slows down, the
brakes heat up by 3°C. A car is traveling 40 km per
hour. The brakes start out at 25°C. How hot will they
be after the car comes to a complete stop?
A car’s engine has many fast-moving metal parts that
create a lot of friction. Oil helps lubricate these parts
and keeps them running smoothly.
The moon Io (EYE-oh)
orbits the planet Jupiter.
Jupiter’s gravity is so
strong that it stretches
and squishes Io.
Friction inside Io
generates heat. It
makes so much heat
that Io’s interior is
melted! The heat even
causes volcanic eruptions.
Race cars use high-tech materials in their tires, brakes, and engines
to reduce friction caused by high speeds.
Heat Energy • Science Friction
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Friction in the Atmosphere
Write your answers on separate paper. Use details from
the text as evidence.
You may not notice it, but air causes friction every
time you move. At very high speeds, friction with
the air creates a lot of heat.
1 Where does the energy come from to create
friction, and what does friction give off?
A meteoroid is a bit of rock that falls toward Earth
from space. As a meteoroid hits Earth’s atmosphere,
friction with the air starts to heat it up. The heat is
so great that most meteoroids burn up completely.
2 Based on what you learned on page 5, which
method would you choose if you were cold
and needed to start a fire quickly? Why?
3 Using the information you learned in the section
about friction and machines, how would you
reduce the friction generated by a bicycle when
you pedal it?
Spacecraft must withstand the heat caused by friction
when they exit and reenter Earth’s atmosphere.
Spacecraft have special heat shields that protect
astronauts from the heat of friction. Once a spacecraft
is in space, there is no friction. Why? There is almost
no matter to rub against in space!
4 Choose one example from the book to demonstrate
how friction is helpful when driving a car.
5 Why does a spaceship need a special shield when
it reenters Earth’s atmosphere?
FOCUS Question
How is friction related to heat energy? Using
information from the book, describe an example
from your own life of when heat from friction
can be useful and an example of when
it can be harmful.
heat shield
When meteoroids burn up, they
make streaks of light, better known
as “shooting stars.”
Heat Energy • Science Friction
The outside of the space shuttle
Endeavour protected the astronauts
from extreme heat.
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