Download Chapter 6: Newton`s Laws of Motion

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Science TEKS 8.7 A
Newton’s Laws of Motion
T
he noise is deafening as drivers
gun their engines, waiting for
the signal to start the race. The
signal comes, and the cars speed forward. One car pulls ahead but has to
slow down to make the turn. What
makes the cars move so fast? How do
they change direction? What causes
them to slow down? In this chapter
you will learn how to describe
motion. You’ll also learn about the
laws of motion that made this race
possible, and how they determine the
motion of objects around you.
What do you think?
Science Journal Look at the picture
below with a classmate. Discuss what
this might be or what is happening.
Here’s a clue: You would have to be
high above Earth to see a sunrise that
looks like this. Write your answer or
best guess in your Science Journal.
152
EXPLORE C
ACTIVITY
onsider the countless possibilities for motion in the
universe. Billions of stars orbit around the center
of the galaxy. The wind bounces air molecules from
leaf to leaf in a grove of trees creating a cool breeze.
A roller coaster plummets down a steep hill before
twisting and looping along its tracks. A fly darts about a room faster than
the eye can follow. Just three laws of motion explain why objects move the
way they do and how different types of forces cause these movements.
Observe these laws in this activity.
Observe motion along varying slopes
1. Lean one end of a ruler on top of three
books to form a ramp. Use a plastic
ruler with a groove down the
middle.
2. Tap a marble so it rolls
up the ramp. Measure
how far it rolls up the ramp before stopping and rolling back down.
3. Repeat step 2 using two books, one book, and no books. The same person
should tap the marble each time, trying to keep the force constant.
Observe
What would happen if you tapped the marble on a smooth, flat path? Write
your observations and predictions in your Science Journal.
FOLDABLES
Reading &Study
& Study
Skills
Making a Question Study Fold Asking yourself questions helps
you to stay focused and better understand Newton’s Laws of
Motion when you are reading the chapter.
1. Place a sheet of paper in front of you so the long side is at the top. Fold
the paper in half from the left side to the right side and then unfold.
2. Fold each side in to the centerfold line to divide the paper into fourths.
Fold the paper in half from top to bottom and unfold.
3. Through the top thickness of paper, cut along both of the middle fold
lines to form four tabs as shown. Label each tab Motion,First Law,
Second Law,and Third Law as shown.
4. As you read the chapter, write under the tabs some questions you have
concerning the main ideas in each section.
Motion
cond
Sea
L w
First
Law
Third
Law
153
SECTION
Motion
What is motion?
Contrast distance and
displacement.
■ Define speed, velocity, and
acceleration.
■ Calculate speed, velocity, and
acceleration.
■
Vocabulary
displacement
speed
velocity
acceleration
Motion occurs constantly all around
you.
Figure 1
A hiker might be interested in
the total distance traveled along
a hike, or in the displacement.
Can total distance be greater than
total displacement? Explain.
N
Cars drive past on streets and freeways. The wind swirls and
causes leaves and paper to fly about. Inside you, your heart
pumps and sends blood throughout your body. When walking,
skating, jumping, or dancing, you are in motion. In fact, motion
is all around you. Even massive pieces of Earth’s surface move—
if only a few centimeters per year.
A traveler wants to know the distance between home and
another place. Is it close enough to walk? Will a flight take half
the day? The coach at your school wants to analyze various
styles of running. Who is the fastest sprinter? Who can change
direction easily in a sprint? To answer these questions, you must
be able to describe motion.
Distance and Displacement One way you can describe the
motion of an object is by how it changes position. There are two
ways to describe how something changes position. One way is to
describe the entire path the object travels. The other way is to give
only the starting and stopping points. Picture yourself on a hiking trip through the mountains. The path you follow is shown in
Figure 1. By following the trail, you will walk 22 km but end up
only 12 km from where you started. The total distance you will
travel is 22 km. However, you will end up only
12 km north of where you started. Displacement is
the distance and direction between starting and ending positions. Your displacement is 12 km north.
Finish
Start
0
154
1
2 km
CHAPTER 6 Newton’s Laws of Motion
Figure 2
For position to change, one object
must move relative to another
object. Which object did the person
on the bench move relative to?
Relative Motion Something that is in
motion changes its position. The position
of an object is described relative to another
object, which is the reference point. Suppose you look out the window in the
morning and see a truck parked next to a
tree. When you look out the window later,
the truck is parked further down the street.
How can you tell that the truck has been in motion? The truck
has changed position relative to the tree. The same is true for the
person in Figure 2.
How can you tell whether an object has
changed position?
Speed
When running, you are interested not only in the distance
you travel, but also in how fast you are moving. To describe how
quickly or slowly you are moving, you give your speed. Speed is
the distance traveled divided by the time needed to travel the
distance. In equation form,
distance
speed time
If you run 3 km in 0.5 h, your speed is
Grade 7 TEKS Review
For a review of the Grade 7 TEKS
There is a Relationship Between
Force and Motion, see page 740.
(3 km)
speed 6 km/h
(0.5 h)
If you run farther in the same amount of time, your speed is
greater. The greater your speed is, the faster you move.
SECTION 1 Motion
155
Math Skills Activity
Calculating Speed
MATH TEKS
8.2 A, B; 8.5 A
Example Problem
A hockey puck slides along the ice for 3 s before crossing the
goal line 6 m away. What is the average speed of the puck before it
crossed the goal line?
Solution
This is what you know:
distance = 6 m
time = 3 s
This is what you need to know:
speed
This is the equation you need to use: speed = distance/time
Substitute the known values:
speed = 6 m/3 s = 2 m/s
Check your answer by multiplying it by the time. Do you
calculate the same distance that was given?
Practice Problem
A swimmer takes 40 s to swim 100 m. What is his or her average speed?
For more help, refer to the Math Skill Handbook.
Constant Speed If you are riding in a car with the cruise
Figure 3
When you read a speedometer,
you are finding your instantaneous speed. When would your
instantaneous speed be the same
as your average speed?
control on, the speed doesn’t change. In other words, the car
moves at a constant speed. If you are running at a constant
speed of 5 m/s, you run 5 m each second. Whether you give your
speed for the first 5 m you run or the last 5 m, you cover each
5-m interval in 1 s. When you are traveling at a constant speed,
the speed at any instant of time is the same.
Changing Speed For some motion, speed is not constant. If you are riding your bike, you must slow down for
intersections and turns and then pedal faster to resume
your speed. Your average speed for your trip would be the
total distance traveled divided by the time you were riding. If you wanted to know how fast you were going at
just one instant, you would be interested in your instantaneous speed. A car speedometer like the one in Figure 3
shows instantaneous speed—how fast the car is moving
at any moment. When speed is constant, average speed
and instantaneous speed are the same.
156
CHAPTER 6 Newton’s Laws of Motion
Velocity
Sometimes you might be
interested not only in how fast
you are going, but also in the
direction. When direction is
important, you want to know
your velocity. Velocity is the displacement divided by time. For
example, if you were to travel
1 km east in 0.5 h, you would
calculate your velocity as follows.
displacement
velocity time
(1 km east)
v (0.5 h)
v 2 km/h east
Figure 4
Airplane pilots are interested in
their velocity. Who else would be
concerned about their velocity?
Like displacement, velocity includes a direction. Velocity is
important to pilots flying airplanes. They rely on control panels
like the one in Figure 4 because they need to know not only how
fast they are flying, but also in what direction.
Acceleration
Displacement and velocity describe how far, how fast, and
where something is going. You also might want to know how
motion is changing. Is the car going faster and faster, or is it
slowing down? Is a skater changing direction? Acceleration is
the change in velocity divided by the amount of time required
for the change to occur. Recall that velocity includes speed and
direction. If an object changes its speed, its direction, or both, it
is accelerating.
Speeding Up and Slowing Down When you think of an
object accelerating, you might think of it moving faster and
faster. If someone says a car accelerates, you think of it moving
forward and increasing its velocity.
However, when an object slows down, it also is accelerating.
Why? Recall that an object is accelerating when its velocity
changes. Velocity can change if the speed of an object changes,
whether the speed increases or decreases, or if it changes direction. If an object slows down, its speed changes. Therefore, if an
object is speeding up or slowing down, it is accelerating.
Measuring Motion
Procedure
1. Measure a fixed distance
such as the length of your
driveway.
2. Use a watch to measure the
time it takes you to stroll,
rapidly walk, and run this
distance.
Analysis
1. Calculate your average
velocity in each case.
2. Use your results to predict
how long it would take you
to go 100 m by each
method.
SECTION 1 Motion
157
Calculating Acceleration If an object speeds up or slows
down, only the speed of the object is changing. You can then use
the change in speed to calculate the acceleration using the following equation.
(final speed initial speed)
acceleration time
For example, suppose the ball in Figure 5 goes from being at rest to
moving at 29.4 m/s in 3 s. What is its acceleration? The ball’s final
speed is 29.4 m/s, and its initial speed is zero because it started
from rest. The time it took for the changes in speed to occur is 3 s.
According to the formula above, the ball’s acceleration is
Figure 5
When this ball is dropped, it
accelerates downward, speeding
up from 0 m/s to 29.4 m/s in 3 s.
(29.4 m/s 0 m/s)
acceleration 3s
2
9.8 m/s
Math Skills Activity
Calculating Acceleration
MATH TEKS
8.1 A, B; 8.2 A, B
Example Problem
Riding your skateboard, it takes you
12 s to accelerate from 2 m/s to 8 m/s.
Find your acceleration.
Solution
initial speed 2 m/s
final speed 8 m/s
time 12 s
This is what you need to know:
acceleration: a
This is the equation you need to use: a (final speed initial speed)/time
Substitute the known values:
a (8 m/s 2 m/s)/12 s
a (6 m/s)/12 s 0.5 m/s2
Check your answer by multipling your answer by the time. Add the initial speed.
Do you calculate the same final speed that was given?
This is what you know:
Practice Problem
A horse canters at 11 m/s. It takes 3 s to reach a gallop of 17 m/s. Find
the acceleration.
For more help, refer to the Math Skill Handbook.
158
CHAPTER 6 Newton’s Laws of Motion
Turning When an object turns or changes direction, its veloc-
Figure 6
ity changes. This means that any object that changes direction is
accelerating. To help understand acceleration, picture running a
race, as shown in Figure 6. As soon as the race begins, you start
from rest and run faster. You speed up, therefore, you accelerate.
When you follow the turn in the track, you are changing your
direction. Because you turn, your velocity changes and you
accelerate. After you pass the finish line, you slow down to a
steady walk. Because your speed changes, you accelerate again.
When the race starts, the runner accelerates by speeding up.
When rounding the corners of
the track, the runner accelerates
by turning.
After crossing the
finish line, the runner accelerates
by slowing down.
What are three ways to accelerate?
Section
Assessment
1. Compare and contrast displacement,
velocity, and acceleration.
2. You walked 100 m forward, then 35 m
backward. What distance did you walk?
3. In question 2, what is your displacement?
4. Why is turning a form of acceleration?
5. Think Critically When you ride in a train
that passes a train moving in the opposite
direction, the other train appears to rush
past you at a speed greater than if you were
watching from a parked car at a crossing.
How does the speed of your train affect
your impression of the passing train’s
speed?
6. Comparing and Contrasting You and a
friend swim out a distance 40 m and swim back
40 m. Your friend lands on his starting spot, but
you land 6 m north of your starting point. If you
both swam for 60 s, compare and contrast total
distance, displacement, and average speed.
For more help, refer to the Science Skill
Handbook.
7. Solving One-Step Equations A car is moving
forward at 12 m/s. It takes 4 s to come to a stop.
Calculate the car’s average acceleration. Include
the direction of the acceleration. For more help,
refer to the Math Skill Handbook.
SECTION 1 Motion
159
SECTION
Newton’s First Law
Laws of Motion
Define force.
Describe Newton’s first law of
motion.
■ Contrast balanced and unbalanced
forces.
■
■
Vocabulary
force
first law
of motion
balanced forces
unbalanced forces
A picture might tell a thousand words, but sometimes it
takes a thousand pictures to show a motion. Look at the strobe
photo in Figure 7. The smoothness of the overall motion is the
result of many individual motions—hands move and legs flex.
Some trainers who work with athletes use such films to help
them understand and improve performance.
What causes such complex motions to occur? Each individual motion can be explained and predicted by a set of rules that
were first stated by Isaac Newton. These rules are called Newton’s laws of motion.
Force An object’s motion changes in response to a force. A
Forces cause motion to change.
Figure 7
A gymnast’s motion can be
explained by Newton’s laws of
motion.
160
force is a push or a pull. A force has a size and a direction—both
are important in determining an object’s motion. For example,
pushing this book on one side will slide it across the desk in the
same direction. Pushing it harder will make it move faster in the
direction of your push. However, if you push the book straight
down, it will not move. The force you exert on the book when
you push is called a contact force. A contact force is a force that
is exerted when two objects are touching each other.
Long-Range Forces However, a
force can be exerted if two objects are
not in contact. If you bring a magnet
close to a paper clip, the paper clip
moves toward the magnet, so a force must be acting on the
paper clip. The same is true if you drop a ball—the ball will fall
downward, even though nothing appears to be touching it. The
forces acting on the paper clip and the ball are long-range
forces. These forces can cause an object’s motion to change
without direct contact. Long-range forces include gravity, magnetism, and electricity.
When scientists need to measure force, they use the newton,
which is abbreviated N and named for Isaac Newton. One newton is about the amount of force needed to lift half a cup of water.
Figure 8
If you were to stop the cart
suddenly, the boxes would
keep moving.
Newton’s First Law of Motion
When you are riding in a car and it comes to a sudden stop,
you feel yourself continuing to move forward. Your seat belt
slows you down and pulls you back into your seat. This can be
explained by the first law of motion—An object will remain at
rest or move in a straight line with constant speed unless it is
acted upon by a force. For a long time it was thought that all
objects come to rest naturally. It seemed that a force had to be
applied continually to keep an object moving. Newton and others theorized that if an object already is moving, then it will continue to move in a straight line with constant speed. For the
object to slow down, a force has to act on it.
Think about what keeps you in your seat when a car comes
to a sudden stop. You can feel yourself moving forward, but the
seat belt applies a force to pull you back and stop your motion.
If you were to push a cart with boxes on it as in Figure 8 and
you suddenly stopped the cart, the boxes would continue moving and slide off the cart.
In a car crash, a car comes
to a sudden stop. According to Newton’s first law,
passengers inside the car
will continue to move
unless they are held in
place. Research the various
safety features that have
been designed to protect
passengers during car
crashes. Write a paragraph
on what you’ve learned in
your Science Journal.
SECTION 2 Newton’s First Law
161
Figure 9
Inertia and Mass The first law of motion is sometimes
The more massive the body is,
the more inertia it has.
It is
easy to speed up or slow down
a small child in a wagon.
Changing the motion of a
wagon carrying a larger child is
harder. How does inertia affect
collisions between cars?
called the law of inertia. Inertia measures an object’s tendency to
remain at rest or keep moving with constant velocity. Inertia
depends on the mass of the object. The more mass an object
has, the more inertia it has and the harder it is to change the
motion of the object. Look at Figure 9. It is easy to get a wagon
to start moving if only one small child is sitting in it. You must
exert more force to start the wagon moving if it holds more
mass. The same is true if you try to stop the wagon after it is
moving. You need to exert a larger force to stop the wagon that
has more mass.
What is inertia?
Adding Forces
Research Visit the
Glencoe Science Web site at
tx.science.glencoe.com
for information about how
airplanes land on aircraft
carriers. These airplanes have
a lot of inertia due to their
large mass and require a lot of
force to bring them to a stop
on the short runway. Communicate to your class what you
learn.
162
According to Newton’s first law, the motion of an object
changes only if a force is acting on the object. Sometimes more
than one force acts on an object, as when several people push a
stalled car to the side of the road. Motion depends upon the size
and direction of all the forces.
If two people push in opposite directions on a box with an
equal amount of force, the box will not move. Because the forces
are equal but in opposite directions, they will cancel each other
out and are called balanced forces. When forces on an object
are balanced, no change will occur in the object’s motion
because the total force on the object is zero.
If one force pushing on the box is greater than the other, the
forces do not cancel. Instead, the box will move in the direction
of the larger force. Forces acting on an object that do not cancel
are unbalanced forces. The motion of an object changes only if
the forces acting on it are unbalanced. The change in motion is
in the direction of the unbalanced force.
CHAPTER 6 Newton’s Laws of Motion
Changes in Motion and Forces
How do unbalanced forces affect the
motion of an object? Look at the
dancer in Figure 10. When she jumps,
she pushes off with a force greater
than the force of gravity pulling her
down. This creates an unbalanced
force upward, and the dancer moves
upward. After she has left the ground
and her feet are no longer in contact
with the floor, gravity becomes the
only force acting on her. The forces on
her are unbalanced and her motion
changes—she slows down as she rises
into the air and speeds up as she
returns to the ground.
The motion of an object changes
when unbalanced forces act on the
object. Recall that if the motion of an
object changes, the object is accelerating. The object can speed
up such as when the dancer in Figure 10 comes back down to
the stage. It might slow down—think of a parachutist after the
chute has opened. The object might turn, as when a bicyclist is
hit by a sudden crosswind. In all cases, an object acted on by an
unbalanced force changes velocity.
Section
Force
due to
gravity
Figure 10
Because the motion of this
dancer is changing, forces
are unbalanced.
Assessment
1. What does the first law of motion say
about the motion of an ice-skater sliding
across the ice at constant velocity?
2. The force of gravity is pulling you toward
the ground. Is this a balanced or an unbalanced force? Explain.
3. What is a force? Give an example.
4. A dropped ball accelerates toward the
ground. Does an unbalanced force act on
the ball? Explain.
5. Think Critically Will a pick-up truck carrying a refrigerator be easier or harder to
turn than the same truck with no load?
Explain, using the principle of inertia.
6. Recognizing Cause and Effect An object in
motion at constant velocity is an effect. What is
the cause? For more help, refer to the Science
Skill Handbook.
7. Communicating Natural philosophers studied
motion for many years before discovering the
first law of motion. Some thought that an
object’s natural state was rest, which all objects
in motion tried to attain. Explain why people
might have thought this and what the first
law of motion says about an object’s natural
state. For more help, refer to the Science
Skill Handbook.
SECTION 2 Newton’s First Law
163
SECTION
Newton’s Second Law
The Second Law of Motion
Predict changes in motion using
Newton’s second law.
■ Contrast different types of friction.
■
Vocabulary
second law of motion
friction
Newton’s second law explains and
predicts how motion will change.
Newton’s first law of motion can help predict when an object’s
motion will change. Can you predict what the change in motion
will be? You know that when you kick a ball, as in Figure 11, your
foot exerts a force on the ball, causing it to move forward and
upward. The force of gravity then pulls the ball downward. The
motion of the ball can be explained by Newton’s second law of
motion. According to the second law of motion, an object acted
on by an unbalanced force will accelerate in the direction of the
force, according to the formula
force
acceleration mass
If a stands for the acceleration, F for the force, and m for the
mass, this formula can also be written as follows.
F
a m
If more than one force acts on the object, the force in this formula
is the combination of all the forces, or the total force that acts on
the object. What is the acceleration if the total force is zero?
In what direction does an object accelerate
when acted on by an unbalanced force?
Figure 11
When you kick a ball, you apply
a force to it. The ball moves in
the direction of the applied force.
Applied
force
164
Using the Second Law
How can Newton’s second law tell you about changes in
motion? Recall that an object accelerates when a change in
motion occurs. The second law allows you to calculate acceleration. For example, if a 10-N force acts on a 2-kg object, what
acceleration does it give the object?
force
acceleration mass
(10 N)
a (2 kg)
a 5 N/kg, or 5 m/s2
Research Visit the
Glencoe Science Web site at
tx.science.glencoe.com for
information about how much
acceleration the space shuttle
and other space vehicles must
have in order to safely escape
Earth’s atmosphere. Communicate to your class what you
learn.
If you know the mass and acceleration of an object, you can use
Newton’s second law to find the force. If both sides of the above
equation are multiplied by mass, m, the equation becomes
F = ma
How do you find force if you are given mass and acceleration?
Math Skills Activity
Calculating Force Given Mass and Acceleration
MATH TEKS
8.1 A, B;
8.2 A, B
Example Problem
A child has a mass of 71 kg. Her bike has a mass of 9 kg. The bike
and rider are accelerated at a rate of 3.20 m/s2. Find the force used to
accelerate the bike and the rider.
Solution
acceleration: a 3.20 m/s2
mass: m 71 kg 9 kg 80 kg
This is what you need to find:
Force: F
This is the equation you need to use: F ma
Substitute the known values:
F (80 kg) (3.20 m/s2) 256 N
Check your answer by solving the original equation (F ma) for m.
Then substitute F and a. Do you calculate the same mass that was given?
This is what you know:
Practice Problem
You are lowering a 12-kg bucket with an acceleration of 0.5 m/s2.
Find the force being used.
For more help, refer to the Math Skill Handbook.
SECTION 3 Newton’s Second Law
165
The Force of Gravity What is the force of gravity on you?
The weight of an object is the force of gravity on the object.
Near Earth’s surface, the force of Earth’s gravity causes all
objects to fall with the same acceleration: 9.8 m/s2. This is
shown in Figure 12. When you are falling, the force of gravity on
you is
F m (9.8 m/s2)
Even if you are standing on the ground, Earth still is pulling on
you with this force. You are at rest because the ground exerts an
upward force that balances the force of gravity.
Because your weight is the force of gravity on you, your
weight is given by the formula
weight mass acceleration due to gravity
Suppose your mass were 60 kg. On Earth, your weight would
be
weight (60 kg) (9.8 m/s2)
weight 588 N
As your mass increases, your weight on Earth also increases.
Mass and Weight Weight and mass are sometimes spoken
Figure 12
The strobe light flashes on and
off at a steady rate. The ball’s
speed is increasing steadily at
9.8 m/s2. The distance it falls in
each flash also increases steadily.
of as the same thing, but the two concepts are different. Weight
is a force, just like a push of your hand is a force. Weight can
change when the force of gravity changes, as it does when astronauts leave Earth’s surface. For example, your weight on the
Moon is different than your weight on Earth. The force of gravity at the Moon’s surface is only about 16 percent as large as the
force of Earth’s gravity. So your weight on the Moon is less than
your weight on Earth. Mass, on the other hand, is a measure of
the amount of matter in an object. It is constant no matter
where you are. An astronaut would have the same mass on Earth
as on the Moon.
How are mass and weight different?
Recall that mass is a measure of inertia, which is how hard it
is to change the motion of an object. Think of how hard it
would be to stop a 130-kg football player rushing at you. Now
suppose you were in outer space. If you were far enough from
Earth, the force of Earth’s gravity would be weak. Your weight
and the football player’s weight would be small. If he were moving toward you, he would be just as hard to stop as on Earth. He
still has the same mass and the same inertia, even though he is
nearly weightless.
166
CHAPTER 6 Newton’s Laws of Motion
Friction
Rub your hands together. You can feel a force between your
hands, slowing their motion. If your hands are covered in lotion,
making them slippery, the force is weaker. If anything gritty or
sticky is on your hands, this force increases. Friction is a force
that resists motion between sufaces that are touching.
Friction always is present when two surfaces of two objects
slide past each other. To keep the objects moving, a force has to
be applied to overcome the force of friction. However, friction is
also essential to everyday motion. How hard would it be to
move without friction? When you walk up a ramp, friction prevents you from sliding to the bottom. When you stand on a
skateboard, friction keeps you from sliding off. When you ride a
bike, friction enables the wheels to propel the bike forward. Friction sometimes can be reduced by adding oil or grease to the
surfaces, but it always is present. There are several types of friction, and any form of motion will include one or more of them.
Static Friction Gently push horizontally on this book. You
should be able to push against it without moving it. This is
because of the static friction between the cover of the book and
the top of the desk. Static friction hinders a stationary object
from moving on a surface when a force is applied to the object.
When you stand on a slight incline, you don’t slide down
because of the static friction between your shoes and the
ground.
When you carry a tray of food to a table and stop to sit
down, Newton’s first law predicts that food on the tray will keep
moving and slide off the tray. Happily, static friction keeps the
food on the tray, allowing you to sit down and enjoy your meal.
This is similar to the example shown in Figure 13.
Figure 13
The rubber mat increases the
static friction acting on the
computer. This static friction
keeps the computer from sliding.
Without static friction, what would
happen when the cart stopped?
Computer
Rubber mat
Cart
SECTION 3 Newton’s Second Law
167
Sliding Friction Push on your book again. Notice
that once the book is in motion and leaves your hand, it
comes to a stop. Sliding friction is the force that slowed
the book down. Sliding friction occurs when two surfaces slide past each other. To keep the book moving,
you have to keep applying a force to overcome sliding
friction.
The friction between a skier and the snow, a sliding
baseball player and the ground, and your shoes and the
skin where a blister is forming are examples of sliding
friction. When you apply the brakes to a bike, a car, or
the skateboard shown in Figure 14, you use sliding friction to slow down.
Rolling Friction A car stuck in snow or in mud spins
its wheels but doesn’t move. Rolling friction makes a
wheel roll forward or backward. If the rolling friction is
large enough, a wheel will roll without slipping. The car
that is stuck doesn’t move because mud or snow makes
the ground too slippery. Then there is not enough
rolling friction to keep the wheels from slipping.
Because rolling friction is the force that enables a wheel to
roll past a surface, the force of rolling friction is in the same
direction that the wheel is rolling. If the wheel is rolling forward,
the rolling friction force also points forward, as shown in Figure
15. Some of the ways that static, sliding, and rolling friction are
useful to a bike rider are shown in Figure 16.
Figure 14
Sliding friction between the
rider’s shoe and the sidewalk
slows the skateboarder down.
What kind of friction is used
when the skateboarder pushes
off to start moving?
Motion of tire
Figure 15
Rolling friction is needed for
this wheel to roll forward.
Without friction, it would
slide along the street.
168
CHAPTER 6 Newton’s Laws of Motion
Rolling friction
VISUALIZING FRICTION
Figure 16
riction is a force that opposes motion. But sometimes friction
can be used to your advantage. For example, you could not ride
a bicycle without friction. A: The rolling friction between the
tires and the pavement pushes the bottom of the tires forward; this
helps rotate the tires and propels the cyclist forward. B: Brake pads
push against the rim of the tire; this creates sliding friction, which
slows the wheel. C: Bicycle pedals have a rough surface that increases static friction and keeps the cyclist’s feet from slipping off.
F
A
B
C
SECTION 3 Newton’s Second Law
169
Air Resistance Do you know why it
Gravity
Air
resistance
Figure 17
Gravity and air resistance are the
forces acting on a parachutist.
When the forces balance each
other, the parachutist falls at a
constant velocity.
is harder to walk in deeper water than in
shallow water? It’s because the deeper
the water is, the more of it you have to
push out of your way to move forward.
You usually do not realize it, but the
same is true for walking on dry land. To
move forward, you must push air out of
your way. Unless you’re in a hurricane, it
is much easier to walk through air
because the molecules in air are farther
apart than water molecules are.
Molecules in air collide with the
forward-moving surface of an object,
slowing its motion. This is called air
resistance. Air resistance is less for a narrow, pointed object than
for a large flat object. Air resistance also increases as the speed of
an object moving through the air increases. Because air resistance is a type of friction, it acts in a direction opposite to the
motion of an object through the air.
Before the sky diver in Figure 17 opens the parachute, his air
resistance is small. The force of air resistance is upward, but is
not large enough to balance the downward force of gravity. As a
result the sky diver falls rapidly. When he opens his parachute
the air resistance is much greater because the parachute has a
large surface area. Then the force of air resistance is large
enough to slow his fall and balance the force of gravity.
Section
Assessment
1. A ball has a mass of 2 kg. What is the force
due to gravity on the ball (its weight)?
2. An angled conveyor belt lifts a box to a shipping area. What force keeps the box from
sliding off the belt?
3. Compare the acceleration of an object acted
on by forces of 1 N and 2 N.
4. What factors influence air resistance?
5. Think Critically The following forces
act on an object: 3 N left, 2 N up, 1 N right,
2 N down. What is the total force on the
object? Give the size and direction.
170
CHAPTER 6 Newton’s Laws of Motion
6. Concept Mapping Make a spider concept
map about the types of friction. Include static
friction, sliding friction, rolling friction, and air
resistance. For more help, refer to the Science
Skill Handbook.
7. Using an Electronic Spreadsheet Calculate
the acceleration of a 40-kg sled acted on by a
20 N force. Then,if the sled starts from rest,use a
computer spreadsheet to calculate the speed of
the sled every second. For more help, refer to
the Technology Skill Handbook.
Static and Sliding Friction
S
tatic friction can hold an object in place
when you try to push or pull it. Sliding friction explains why you must continually push on
something to keep it sliding across a horizontal
surface. In this activity, you’ll measure and compare static and sliding friction.
What You’ll Investigate
How do different types of friction compare?
Materials
spring scale
block of wood or other material
tape
Goals
■ Observe static and sliding friction.
■ Measure static and sliding friction.
■ Compare and contrast static and sliding
friction.
Safety Precautions
Procedure
1. Attach a spring scale to the block and set it on
the table. Experiment with pulling the block
with the scale so you have an idea of how
hard you need to pull to start it in motion and
continue the motion.
2. Measure the force needed to start the block
in motion. This is the force of static friction.
3. Measure the force needed to keep the block
moving at a steady speed. This is the force of
sliding friction on the blocks.
4. Repeat steps 2 and 3 on a different surface,
such as carpet. Record your measurements in
your Science Journal.
Conclude and Apply
1. Compare the forces of static friction and
sliding friction on both horizontal surfaces.
Which force is greater?
2. On which horizontal surface is the force of
static friction greater?
3. On which surface is the force of sliding friction
greater?
4. Which surface is rougher? How do static and
sliding friction depend on the roughness of
the surface?
5. Explain how different materials affect the
static and sliding friction between two
objects.
Compare your conclusions with those of other
students in your class. For more help, refer
to the Science Skill Handbook.
ACTIVITY
171
SECTION
Newton’s Third Law
Action and Reaction
Interpret motion using Newton’s
third law.
■ Analyze motion using all three
laws.
■
Vocabulary
third law of motion
Newton’s three laws together will
help you understand motion.
When you push off the side of a pool, you accelerate. By
Newton’s laws of motion, when you accelerate, a force must be
exerted on you. Where does this force come from?
Newton’s first two laws explain how forces acting on a single
object affect its motion. The third law describes the connection
between the object receiving a force and the object supplying
that force. According to the third law of motion, forces always
act in equal but opposite pairs. This often is restated—for every
action, there is an equal but opposite reaction. For example, if
object A exerts a force on object B, then object B exerts a force of
the same size on object A.
What relationship does the third law of motion
describe?
Figure 18
Notice that when you walk, you
push backward on the ground,
yet you move forward. This is
because when you push back on
the ground, the ground exerts an
equal force forward on you.
Your
force
172
The Third Law in Action Newton’s third law means that
when you lift your book bag, your book bag pulls back. When
you jump, you push down on the ground, which pushes up on
you. When you walk, you push back on the ground, and the
ground pushes forward on you. When you throw a ball, you
push forward on the ball and it pushes
back on your hand. Figure 18 shows
that when you exert a force on the floor,
the floor exerts an equal force on you in
the opposite direction.
The same is true for any two
objects, regardless of whether the force
between the objects is a contact force or
a long-range force. For example, if you
Reaction
place two bar magnets with opposite
force
poles facing one another, they will
move toward each other. Each magnet
applies a force to the other, even though
they are not in direct contact. No matter how one object exerts a force on
another, the other object always exerts
an equal force in the opposite direction.
CHAPTER 6 Newton’s Laws of Motion
Figure 19
Static friction between your shoes and the
ground makes it possible to push a heavy door
forward. Without static friction, pushing the
door would make you slide backwards.
How can anything move?
Action and reaction forces are not the same as balanced
forces. Recall that balanced forces are forces that act on the same
object and cancel each other. Action and reaction forces act on
different objects. When you kick a soccer ball, your force on the
ball equals the ball’s force on you. The harder you kick, the
greater the force the ball exerts on your foot. A hard kick can
hurt because the ball exerts a large force on your foot. Unlike
balanced forces, action and reaction forces can cause the motion
of objects to change.
How do balanced forces differ from action and
reaction forces?
Using Friction When you push a heavy door, as in Figure 19,
you exert a force on the door to move it. The door exerts an
equal force back on you. Why don’t you move? When you push
on the door, your feet are touching Earth and static friction
keeps you from sliding. The reaction force is exerted on you and
Earth together. The door can’t push enough on you and Earth to
move you back. If you wear slippery shoes or if the ground is
slippery, you might not be able to open a heavy door. The force
exerted by the door on you would cause your feet to slip out
from under you. Static friction hinders you from moving when
you throw a ball or pull a book off a shelf. However, if you throw
a heavy ball while standing on ice, you will slide in the opposite
direction to the direction you threw the ball.
SECTION 4 Newton’s Third Law
173
Doing the Math If a 50-kg person in the middle of an ice-
Observing the Laws of
Motion
Procedure
1. Incline one end of a board to
a height of 20 cm.
2. Lay a meterstick on the floor
with one end flush to the
board, and place a softball
where the board and meterstick meet.
3. Hold a baseball 10 cm up the
slope and roll it into the softball. Measure the distance
the softball is pushed.
4. Repeat step 3 from different
heights.
skating rink pushes a 20-kg box with a force of 10 N, what will
the acceleration of each be? According to Newton’s third law, the
forces are equal. Use the second law to find the accelerations.
Find the acceleration of the box.
force
acceleration mass
(10 N)
a (20 kg)
a 0.5 m/s2
Find the acceleration of the person.
force
acceleration mass
(10 N)
a (50 kg)
a 0.2 m/s2
Analysis
1. Compare the distances the
baseball pushed the softball.
2. Use the laws of motion to
explain your results.
If the same force is applied to two different objects, the one with
the larger mass has a smaller acceleration.
Figure 20
the ground. According to the third law, you aren’t just pulled
toward Earth—Earth also is pulled toward you. When you jump
into a swimming pool, how far does Earth move? The force you
exert on Earth is the same as the force Earth exerts on you.
However, Earth is trillions of times more massive than you are.
Because Earth has such a large mass, the pull you exert on it isn’t
noticeable.
The motion of these galaxies
can be explained by Newton’s laws of motion.
Gravity and the Third Law Gravity is pulling you down to
Newton’s laws of
motion apply to all
objects, including
the distant galaxies shown in Figure 20. Just as
the Sun exerts a gravitational force on Earth,
Earth exerts an equal and opposite force on the
Sun. This force causes a small variation in the
Sun’s motion. Variations like this can be used to
find planets outside Earth’s solar system. These
planets do not reflect enough light to be
observed directly by scientists on Earth. By
observing small variations in the motions of a
star, they are able to determine whether a star is
being affected by the force of an orbiting planet.
174
Combining the Laws
The laws of motion describe how an object moves
when forces act on it. The object might be a football, a
planet, or a bone in your arm. Consider what happens
during a jump, as shown in Figure 21.
First, you push down on the ground. According to
the third law of motion, the ground pushes up on you
with an equal and opposite force. The force you exert
on the ground does not cause a noticeable acceleration
to Earth. However, the force the ground exerts on you
gives you an acceleration upward. Two forces act on
you—gravity pulls you down, and the force from your
jump pushes you up. The overall force is upward. As
the second law predicts, you accelerate upward as your
foot pushes against the ground.
When your feet leave the ground, gravity is the only
force acting on you. Again according to the second law,
you accelerate in the direction of this unbalanced
force. The downward acceleration slows you until you
stop at the top of your jump and then causes you to
increase your speed downward until you reach the ground.
When your feet hit the ground, the ground exerts an upward
force on you. The force must be greater than the downward
force of gravity to slow you down. When you stop accelerating,
all the forces on you are balanced. As the first law predicts, you
remain at rest.
Section
Figure 21
A simple jump includes all of
Newton’s laws. What forces are
acting in each part of this jump?
How do they affect your motion?
Assessment
1. What are the action and reaction forces on
this book when it rests on your hands?
2. You jump from a boat and the boat moves
back as you move forward. Explain.
3. Explain how action and reaction forces
differ from balanced forces. Use an
example.
4. Suppose you face a child, half your mass, on
ice. If you push the child, who will have the
greater acceleration? By how much?
5. Think Critically When you hit a baseball
with a bat, what does the ball exert a force
on? What change in motion does this cause?
6. Comparing and Contrasting Compare and
contrast the first two laws of motion with the
third law of motion. For more help, refer to the
Science Skill Handbook.
7. Solving One-Step Equations Your mass is
60 kg. A skateboard’s mass is 3 kg. If you stand
on the ground and push the skateboard away
with a force of 6 N, what is the force on the
board? Does it accelerate? How fast? What is
the force of the board on you? Do you accelerate? If so, how fast? For more help, refer to
the Math Skill Handbook.
SECTION 4 Newton’s Third Law
175
Balanced and Unbalanced Forces
N
ewton’s laws tell you that to change the velocity of an object, there must be
an unbalanced force acting on the object. Changing the velocity can involve
changing the speed of the object, changing the direction of motion, or changing
both. How can you apply an unbalanced force to an object? How does the motion
change when you exert a force in different ways?
Recognize the Problem
What types of forces will change the speed of an object’s motion, the direction, or both?
Thinking Critically
How could you exert a force on a block to affect its speed? Its direction?
Goals
Possible Materials
■ Describe how to create balanced
block
book
and unbalanced forces on an object.
■ Demonstrate forces that change
the speed and the direction of an
object’s motion.
Safety Precautions
176
CHAPTER 6 Newton’s Laws of Motion
strings (2)
spring scales (2)
Data Source
Go to the
Glencoe Science Web site at
tx.science.glencoe.com for data
from other students.
Planning the Model
1. Choose an appropriate object and
describe how you plan to exert
forces on the object.
2. List several different ways to exert
forces or combinations of forces on
the object. Think about how strong
each force will need to be to
change the motion of the object
you will use. Include at least one
force or combination of forces that
you think will not change the
object’s motion.
3. When are the forces balanced and
when are they unbalanced? Predict
which forces will change the object’s
direction, its speed, both, or neither.
Check the Model Plans
1. Make sure that your teacher approves
your plans before going any further.
2. Compare your plans for exerting
forces with those of others in your
class.Discuss why each of you chose
the forces you chose.
Making the Model
1. Set up your model so that you can
2. Exert each of the forces in turn and
exert each of the forces that you
listed.
record how it affects the object’s
motion.
Analyzing and Applying Results
1. For each of the forces or combinations of forces that you applied to
the object, list all of the forces acting on the object. Was the number
of forces acting always the same?
Was there a situation when only a
single force was being applied?
Explain.
2. What happened when you exerted
balanced forces on the object? Were
the results for unbalanced forces the
same for different combinations of
forces? Why or why not?
3. Were your predictions correct?
4. Which of Newton’s laws of motion
did you demonstrate in this activity?
5. Suppose you see a pole that is supposed to be vertical, but is starting
to tip over. What could you do to
prevent the pole from falling over?
Describe the forces acting on the
pole as it starts to tip and after
you do something. Are the forces
balanced or unbalanced?
Compare your results with those of other
students in your class. Discuss how different
combinations of forces affect the motion of
the objects.
ACTIVITY
177
SCIENCE AND
Society
SCIENCE
ISSUES
THAT AFFECT
YOU!
I
f you’ve been to an amusement park lately,
you’ve seen the change in roller coasters.
They’re taller and faster. They swoop
through curves and corkscrews, flipping
riders upside down and sideways. But while
roller coasters are more popular than ever,
there are more concerns about their safety.
Should there
be a limit to
the size and
speed of roller
coasters?
Bigger
,
178
Dangerous Coasters?
There was a sharp rise in the number of
amusement park ride injuries during the late
1990s. In 1998 alone, 4,500 injuries (many
on coasters) required emergency treatment.
This coincides with a time when roller coasters were getting bigger and faster.
You can now ride a 30-story-high roller
coaster that will drop you downhill at speeds
approaching 160 km/h. Some people wonder
if high-speed drops and twists put unhealthy
stress on the body. “Technology and ride
design are outstripping our understanding of
the health effects of high forces on riders,”
said one lawmaker.
Skeptics argue that, even with safety
measures, accidents on super coasters are
bound to be severe because of their speed
and height. So there must be a limit on how
high and fast they can be.
Safe As Can Be
Supporters of the new roller coasters say
that, although they receive lots of attention,
injuries and deaths are rare when you consider the hundreds of millions of annual rides
taken.
What’s more, most accidents or deaths
result from breakdowns or foolish behavior of
riders, not bad design.
Experts say the rides are safer now
because they’re designed and evaluated by
computers. And, they say the ultimate safeguards are Newton’s laws of motion. Designers use these laws to figure out how fast a
roller coaster should move around a turn,
how tight to make the turn, and how much to
bank the coaster tracks to safely balance the
forces acting on the rider. When a coaster
does a loop, designers want the ride to feel
dangerous. But because of the planned
speed of the ride and curvature of the track,
designers can use the forces of motion and
gravity to keep passengers safely in place.
Supporters of super coasters also point
out that safety depends on following rules.
It’s safe to whip around a curve in a rollercoaster car when seated, for example. But
standing to horse around might throw your
center of gravity above the car’s side, pitching you out.
With amusement parks drawing hundreds of millions of visitors yearly, super
coasters are not going away. But with accidents increasing, people who design and
who ride coasters must consider both safety
and thrills.
CONNECTIONS Research Choose an amusement park ride. Do
research on the Glencoe Science Web site to find out how forces act on you
while you’re on the ride to give you thrills, but still keep you safe. Write a
report with diagrams showing how the forces work. Share it with the class.
For more information, visit
tx.science.glencoe.com
Chapter
XX
6 Study Guide
Section 1 Motion
1. Motion occurs when an object changes its
position. The position of an object is
described relative to a reference point.
2. Distance is the entire path length an object
travels. Displacement is the distance and
direction from a starting point to an ending
point.
3. Speed is the distance divided by the time.
Velocity is the displacement divided by the
time. Acceleration is the change in velocity
divided by the time. An object is accelerating if its speed or direction of motion
changes. Which of these football players is
accelerating?
3. The first law of motion states that an
object will remain at rest or in motion at
constant velocity until it is acted upon by
an unbalanced force.
Section 3 Newton’s Second Law
1. The second law of motion states that an
object acted on by an unbalanced force
will accelerate in the direction of the
force according to the formula
acceleration force/mass.
2. An object acted on by an unbalanced force
can speed up, slow down, or turn.
3. Friction is a force that resists motion
between two surfaces that are touching.
Types of friction include static, sliding, and
rolling friction. Air resistance acts against
objects moving through the air.
Section 4 Newton’s
Third Law
Section 2 Newton’s First Law
1. A force is a push or a pull.
2. Balanced forces cancel each other.
Unbalanced
forces cause an
object’s motion
to change. Are
the forces on
this dancer
balanced or
unbalanced?
Explain.
180
CHAPTER STUDY GUIDE
1. The third law states
that forces always act
in equal but opposite
pairs. What are action
and reaction forces in
this picture?
2. Action and reaction
forces do not cancel
because they act on
different objects.
FOLDABLES
After You Read
Use the information on
your Foldable and in
the chapter to explain
Newton’s three laws of motion.
Reading &Study
& Study
Skills
Chapter
XX
6 Study Guide
Complete the following table on the laws of motion.
Laws of Motion
Law
First
Statement
motion of object
and force on object
Vocabulary Words
acceleration
balanced forces
displacement
first law of motion
force
friction
Third
An object will remain at
rest or in motion at constant
velocity until it is acted
upon by an unbalanced force.
Describes Relation
Between What?
a.
b.
c.
d.
e.
f.
Second
Using Vocabulary
g. second law of
motion
h. speed
i. third law of
motion
j. unbalanced forces
k. velocity
Study Tip
Draw pictures of difficult concepts described in
this chapter. Then label each picture with its
concept. This will help you remember the concept
more easily.
For each set of vocabulary words below,
explain the relationship that exists.
1. displacement and velocity
2. force and third law of motion
3. speed and velocity
4. force and friction
5. unbalanced forces and second law
of motion
6. balanced forces and first law of
motion
7. acceleration and second law of motion
8. acceleration and force
CHAPTER STUDY GUIDE
181
Chapter
15
6 Assessment &
Choose the word or phrase that best answers
the question.
1. Which does NOT change when an unbalanced force acts on an object?
A) speed
C) mass
B) velocity
D) motion
2. What force holds you to a sled when it starts
moving?
A) sliding friction
C) air resistance
B) static friction
D) rolling friction
3. Which would include a direction?
A) mass
C) velocity
B) inertia
D) distance
4. What does inertia measure?
A) speed
C) mass
B) force
D) acceleration
5. The unbalanced force on a football is 2 N
downward. Which way does it accelerate?
A) upward
B) downward
C) doesn’t accelerate
D) depends on initial velocity
6. A 50-N force acts on a 4-kg object. What is
the acceleration of the object?
A) 200 m/s2
B) 25 m/s2
C) 12.5 m/s2
D) the same as its velocity
7. Which of the following indicates that the
forces on an object are balanced?
A) It is slowing down.
B) It is speeding up.
C) It moves at constant velocity.
D) It turns.
8. Which type of friction is needed to run?
A) static friction
C) rolling friction
B) sliding friction
D) air resistance
182
CHAPTER ASSESSMENT
Review
9. Carol walks at 0.6 m/s for 60 s. How far
does she go?
A) 10 m
C) 100 m
B) 36 m
D) 360 m
10. The action force is 3 N left. What is the
reaction force?
A) 0 N left
C) 3 N right
B) 3 N left
D) 6 N right
11. Explain why gymnasts and climbers put
chalk on their hands.
12. Give two examples of rolling friction.
13. A 500-N force is applied to a 10-kg object
and a 5-kg object. Which one accelerates
faster? Explain.
14. You are skiing down a hill at constant speed.
What do Newton’s first two laws say about
your motion?
15. How can an object be moving if there is no
unbalanced force acting on it?
16. Comparing and Contrasting When you
come to school, what distance do you
travel? Is the distance you travel greater
than your displacement from home?
17. Drawing Conclusions If the gravitational
force between Earth and an object always
causes the same acceleration, why does
a feather drop more slowly than a
hammer does?
18. Recognizing Cause and Effect Why does a
baseball thrown from the outfield to home
plate follow a curved path rather than a
straight line?
Chapter
15
6 Assessment
19. Concept Mapping Complete the concept
maps below for Newton’s laws of motion.
Marlena did some research about
the speed of sound. The information she
gathered is shown in the table below.
Newton’s
First Law
also
known as
TAKS Practice
motion stays
the same
TEKS 8.2 A, C
Speed of Sound in Different Materials
Newton’s
Second Law
acceleration
Newton’s
Third Law
change
in motion
reaction
force
force pair
acts
20. Making Models Design a model to demonstrate air resistance.
Material
Speed (m/s)
Air
335
Water
1,554
Wood
3,828
Iron
5,103
Stone
5,971
Study the table and answer the following
questions.
21. Make a Poster Make a poster identifying all
the forces acting in each of the following
cases. When is there an unbalanced force?
How do Newton’s laws apply?
■ You push against a box; it doesn’t move.
■ You push harder, and the box starts
to move.
■ You push the box across the floor at a
constant speed.
■ You don’t touch the box. It sits on
the floor.
TECHNOLOGY
Go to the Glencoe Science Web site at
tx.science.glencoe.com or use the
Glencoe Science CD-ROM for additional
chapter assessment.
1. According to this information, in
which material is the speed of sound
greater than 5,000 m/s?
A) water
C) air
B) wood
D) stone
2. A reasonable hypothesis based on these
data is that _____ .
F) sound travels at the same speed in
all materials
G) sound travels faster through solids
than through liquids and gases
H) sound does not travel through
gases
J) sound travels faster through liquids
than through solids and gases
CHAPTER ASSESSMENT
183