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Chapter Introduction
Lesson 1 Describing
Motion
Lesson 2 Forces
Lesson 3 Newton’s Laws
of Motion
Chapter Wrap-Up
In what ways do forces
affect an object’s motion?
What do you think?
Before you begin, decide if you agree or
disagree with each of these statements.
As you view this presentation, see if you
change your mind about any of the
statements.
Do you agree or disagree?
1. You must use a reference point to
describe an object’s motion.
2. An object that is accelerating must be
speeding up.
3. Objects must be in contact with one
another to exert a force.
Do you agree or disagree?
4. Gravity is a force that depends on the
masses of two objects and the distance
between them.
5. All forces change the motion of objects.
6. The net force on an object is equal to
the mass of the object times the
acceleration of the object.
Describing Motion
• What information do you need to
describe the motion of an object?
• How are speed, velocity, and
acceleration related?
• How can a graph help you understand
the motion of an object?
Describing Motion
• motion
• speed
• reference point
• velocity
• distance
• acceleration
• displacement
Motion
• A. Motion
• 1. Motion is the process of changing
position.
• 2. The starting point used to describe
the motion or position of an object is
called the reference point.
Motion (cont.)
• 3. Distance is the total length of your
path of motion.
• 4. The distance between your final
position and your starting position
is displacement.
• 5. A complete description of motion
includes a(n) reference point, your
displacement, and your direction.
Motion (cont.)
The distance traveled and the
displacement from the game to the
vending machine differ.
Motion (cont.)
What information do you need
to describe an object’s motion?
Speed
• B. Speed
• 1. Speed is the distance an object
moves divided by the time it took to
move that distance.
• 2. Speed can be constant or changing.
• a. A constant speed is unchanging
because the distance traveled per unit of
time remains the same.
• b. Speed is changing when the distance
traveled per unit of time does not remain
the same.
• c. Average speed is the ratio of the
distance moved and the time it took to
move that distance.
Speed (cont.)
The girl moves with constant speed during
the first 4 seconds. The girl’s speed begins
to change between seconds 4 and 5.
Velocity
• C. Velocity
• 1. The speed and direction of an
object’s motion is the velocity of the
object.
velocity
from Latin velocitatem, means
“swiftness or speed”
Often, velocity is shown by using an arrow.
The length of the arrow represents the
speed of an object, while the direction in
which the arrow points represents the
direction in which the object is moving.
The velocity of an object changes if the
speed changes, the direction changes, or
both the speed and the direction change.
Velocity (cont.)
• 2. Velocity is often represented by using
a(n) arrow.
• a. The total length of the arrow shows
speed.
• b. The direction of the arrow shows the
direction the object moves.
• 3. When an object’s speed and direction
remain the same, the object has
constant velocity.
• 4. Velocity changes when the speed or
the direction of the motion of the object
changes.
• 5. The measure of how quickly an object’s
velocity changes are called acceleration.
Velocity (cont.)
Can an object traveling at a
constant speed have a changing
velocity? Why or why not?
Calculating Acceleration
• D. Calculating Acceleration
• 1. Acceleration can be calculated by
dividing the change in velocity by the
time it took for the velocity to change.
•
Calculating Acceleration (cont.)
Calculating Acceleration (cont.)
How does acceleration differ
from velocity?
Calculating Acceleration (cont.)
• When an object speeds up, its final
velocity is greater than its initial velocity.
• When an object speeds up, it has
positive acceleration.
• When an object slows down, the initial
velocity is greater than its final velocity.
• As an object slows down, it has negative
acceleration, or deceleration.
• E. Using Graphs to Represent Motion
• 1. A(n) displacement-time graph shows
time on the x-axis and displacement
from a(n) reference point on the y-axis.
• a. A line on this type of graph
represents average speed.
• b. The line does not show the actual
path of motion.
• 2. A(n) speed-time graph shows time on
the x-axis and speed on the y-axis.
• a. A point on the line on this type of
graph represents the speed of the object
at that time.
• b. A(n) horizontal line at y = 0 on a
displacement-time graph or a speed-time
graph shows that the object is at rest.
• c. The term constant speed means
average speed.
Using Graphs to Represent Motion
(cont.)
How can a graph help you
understand an object’s motion?
• A description of an object’s motion
includes a reference point, a direction
from the reference point, and a
distance.
• Speed is the distance traveled by an
object in a unit of time. Velocity
includes both speed and direction of
motion.
• Acceleration is a change in velocity.
Velocity changes when either the
speed, the direction, or both the speed
and the direction change.
Which term refers to the distance
between an initial position and a
final position?
A. displacement
B. distance
C. motion
D. speed
Which is the measure of how
quickly the velocity of an object
changes?
A. acceleration
B. distance
C. motion
D. speed
On a speed-time graph, what
does the x-axis show?
A. average speed
B. displacement
C. distance
D. time
Do you agree or disagree?
1. You must use a reference point to
describe an object’s motion.
2. An object that is accelerating must be
speeding up.
Forces
• How do different types of forces affect
objects?
• What factors affect the way gravity
acts on objects?
• How do balanced and unbalanced
forces differ?
Forces
• force
• gravity
• contact force
• balanced forces
• noncontact force
• unbalanced
forces
• friction
What are forces?
• A. What are forces?
• 1. A(n) force is a push or a pull that one
object exerts on another object.
• 2. A force has a size and a(n) direction.
• a. The length of a force arrow relates to
the size of the force.
• b. The direction of a force arrow shows
the direction of the force.
• 3. The unit of force is the newton.
• 4. A force can change an object’s velocity,
which is the speed and direction of an
object, causing acceleration.
Types of Forces
• B. Types of Forces
• 1. A(n) contact force
is a push or a pull on
one object by
another object that is
touching it.
• 2. Contact forces
are also called
mechanical forces.
Steve Cole/Getty Images
Types of Forces (cont.)
• 3. A(n) noncontact
force is a force that
pushes or pulls an
object without touching
it.
• 4. Examples of
noncontact forces
include magnetism and
gravity.
Friction
• C. Friction
• 1. Friction is a contact force that resists
the sliding motion between two objects
that are touching.
• 2. The direction of the force of friction is
opposite the direction of the motion.
• 3. Rougher surfaces produce greater
friction than smoother surfaces do.
Gravity
• D. Gravity
• 1. The noncontact attractive force
existing between all objects with mass
is gravity.
• 2. The measure of the amount of gravity
acting on an object is its weight.
Gravity (cont.)
• 3. The amount of
gravitational force
decreases as the
distance between
two objects
increases; thus, an
astronaut’s weight
decreases as she
or he moves away
from Earth into
space.
• .
• 4. Gravity is also
affected by mass.
Mass is the amount
of matter in an object.
As the amount of
mass increases, the
force of gravity
between two objects
increases.
• a. The effect of the force of gravity is most
noticeable if one object is very massive.
• b. Although the force of gravity acts
equally on two objects, the less massive
object accelerates more quickly.
Combining Forces
• E. Combining Forces
• 1. The net force is the sum of all forces
acting on an object.
• a. When forces act in the same
direction, you can add the forces
together.
• b. If forces act in the opposite direction,
add them just as you add positive and
negative numbers.
Combining Forces (cont.)
When different forces act on an object in
the same direction, you can find the net
force by adding the forces together.
Combining Forces (cont.)
• 2. Forces are balanced or unbalanced.
Combining Forces (cont.)
• a. If the net force acting on an object is
0 N, the forces acting on it are
balanced forces.
Combining Forces (cont.)
• b. Balanced forces do not affect the
motion of an object.
• c. Unbalanced forces are the result of a
net force that does not equal zero that
acts on an object.
• d. The motion of an object changes
when unbalanced forces act on it.
• Forces are pushes and pulls exerted
by objects on each other. Contact
forces occur when objects are
touching. Noncontact forces act from
a distance.
• Gravity is a force of
attraction between
two objects. The
amount of
gravitational force
depends on the
mass of the objects
and the distance
between them.
• Balanced forces do not affect motion.
Unbalanced forces change motion.
Which is a measure of the force
of gravity acting on an object?
A. friction
B. mass
C. unbalanced force
D. weight
Which term refers to the sum of
the forces acting on an object?
A. friction
B. net force
C. balanced forces
D. unbalanced forces
Which force pushes or pulls an
object without touching it?
A. balanced
B. contact
C. net
D. noncontact
Do you agree or disagree?
3. Objects must be in contact with one
another to exert a force.
4. Gravity is a force that depends on the
masses of two objects and the distance
between them.
Newton’s Laws of Motion
• How do unbalanced forces affect an
object’s motion?
• How are the acceleration, the net
force, and the mass of an object
related?
• What happens to an object when
another object exerts a force on it?
Newton’s Laws of Motion
• inertia
• Newton’s first law of motion
• Newton’s second law of motion
• Newton’s third law of motion
• force pair
Newton’s Laws
• Forces are measured in a unit called a
newton (N), named after English scientist
Isaac Newton, who studied the motion of
objects.
• Newton summarized his findings in three
laws of motion.
Newton’s First Law
The tendency of an object to resist a
change in motion is called inertia.
inertia
Science Use the tendency to
resist a change in motion
Common Use lack of action
Newton’s First Law (cont.)
• Newton’s first law of motion states that
if the net force acting on an object is
zero, the motion of the object does not
change.
• Newton’s first law of motion is
sometimes called the law of inertia.
Newton’s First Law (cont.)
• When the forces acting on an object are
balanced, the object is either at rest or
moving with a constant velocity.
• Unbalanced forces can cause an object
to accelerate or decelerate.
• Unbalanced forces can also cause an
object to change direction.
Newton’s First Law (cont.)
accelerate
from Latin celer, means “swift”
Newton’s First Law (cont.)
If one force on an object is
5 N upward and the other is
10N downward, what is the
object’s motion?
Newton’s Second Law of Motion
Newton’s second law of motion states
that the acceleration of an object is equal
to the net force exerted on the object
divided by the object’s mass.
Newton’s Second Law of Motion (cont.)
• The direction of acceleration is the same
as the direction of the net force.
• Newton’s second law lets you predict
what combination of force and mass you
need to get the acceleration you need.
Newton’s Second Law of Motion (cont.)
How are the acceleration, the
net force, and the mass of an
object related?
Newton’s Third Law
• Newton’s third law of motion says that
when one object exerts a force on a
second object, the second object exerts
a force of the same size, but in the
opposite direction, on the first object.
• When two objects apply forces on each
other, one of the forces is called the
action force, and the other is called the
reaction force.
Newton’s Third Law (cont.)
What happens when one object
exerts a force on a second object?
Newton’s Third Law (cont.)
• When two objects exert forces on each
other, the two forces are a force pair.
• Force pairs are not the same as
balanced forces.
• Balanced forces combine or cancel each
other out because they act on the same
object. Each force in a force pair acts on
a different object.
Newton’s Laws in Action
• Newton’s laws do not apply to all motion
in the universe, such as the motion of
very tiny objects, such as atoms or
electrons, or the motion of objects that
approach the speed of light.
• You can often see the effects of all three
of Newton’s laws at the same time.
Newton’s Laws in Action (cont.)
DEX IMAGE/Getty Images
Masterfile
• Newton’s first law of motion states
that the motion of an object remains
constant unless acted on by an
outside force. This also is called the
law of inertia.
• Newton’s second law of motion
relates an object’s acceleration to its
mass and the net force applied to the
object.
• Newton’s third law of motion states
that for every action force, there is an
equal but opposite reaction force.
The two forces are called a force
pair.
When balanced forces act on an
object, the object is either moving
with a constant velocity or _____.
A. speeding up
B. slowing down
C. changing direction
D. at rest
According to Newton’s second law
of motion, the direction of
acceleration is the same as which
direction?
A. the direction of the action force
B. the direction of the inertia
C. the direction of the net force
D. the direction of reaction force
Which is also known as the law of
inertia?
A. Newton’s first law of motion
B. Newton’s second law of motion
C. Newton’s third law of motion
D. a force pair
Do you agree or disagree?
5. All forces change the motion of objects.
6. The net force on an object is equal to
the mass of the object times the
acceleration of the object.
Key Concept Summary
Interactive Concept Map
Chapter Review
Standardized Test Practice
Forces are pushes and
pulls that may change
the motion of an object.
Balanced forces result
in an object remaining
at rest or moving at a
constant speed.
Unbalanced forces
result in the acceleration
of an object.
Lesson 1: Describing Motion
• An object’s motion depends on how
it changes position. Motion can be
described using speed, velocity, or
acceleration.
• Speed is how fast an object moves.
Velocity describes an object’s speed
and the direction it moves.
Acceleration describes the rate
at which an object’s velocity changes.
• A graph can show you how either the displacement
or the speed of an object changes over time.
Lesson 2: Forces
• A force is a push or pull on an object. Contact forces
include friction and applied forces. Noncontact
forces include gravity, electricity, and magnetism.
• Gravity is a force of attraction between any two
objects. Gravitational force increases as the masses
of the objects increase and decreases as the
distance between the objects increases.
• Balanced forces acting on an object cause no
change in the motion of the object. When
unbalanced forces act on
an object, the sum of the
forces is not equal to zero.
Unbalanced forces cause
acceleration.
Lesson 3: Newton’s Laws of Motion
• Inertia is the tendency of an object to resist a
change of motion Newton’s first law of motion states
that an object will remain at rest or in constant
straight-line motion unless unbalanced forces act on
the object.
• Newton’s second law of motion states that the
acceleration of an object increases as the force
acting on it increases and decreases as the mass of
the object increases.
• Newton’s third law of motion states that for every
action force, there is an equal but opposite reaction
force. The action-reaction forces are called a force
pair.
To describe motion you need both
direction and what?
A. acceleration
B. reference point
C. speed
D. velocity
Which describes the distance an
object moves divided by the time
it took to move that distance?
A. acceleration
B. displacement
C. speed
D. velocity
Which term refers to a push or
a pull applied by one object to
another object that is touching it?
A. contact force
B. friction
C. net force
D. noncontact force
Which is a contact force that
resists the sliding motion between
two objects that are touching?
A. friction
B. gravity
C. inertia
D. mass
Which term describes the
tendency of an object to resist a
change in motion?
A. force pair
B. gravity
C. inertia
D. unbalanced forces
Which changes when an
unbalanced force acts on
an object?
A. inertia
B. mass
C. motion
D. weight
Which is the speed and direction
of an object’s motion?
A. acceleration
B. displacement
C. distance
D. velocity
You are riding a bike. In which
situation are the forces acting on
the bike balanced?
A. You pedal to speed up.
B. You turn at constant speed.
C. You coast to slow down.
D. You pedal at constant speed.
Which is a noncontact force that
exists between all objects that
have mass?
A. friction
B. gravity
C. inertia
D. net force
When two objects exert forces
on each other, the two forces
are called what?
A. action force
B. force pair
C. inertia
D. reaction force