In what ways do forces
affect an object’s
motion?
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?
Motion
•  Motion is the process of changing
position.
•  The starting point you use to describe
the motion or the position of an object
is called the reference point.
•  In addition to using a reference point to
describe motion, you also need a
direction.
Motion (cont.)
•  Distance is the total length of the path
traveled.
•  Displacement is the distance between
an initial, or starting, position, and a
final position.
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
•  Speed is the distance an object moves
divided by the time it took to move that
distance.
•  Speed can be constant or changing.
•  Average speed is the ratio of the
distance an object moves to the time it
takes for the object 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
Velocity is the speed and direction of an
object’s motion.
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.
Each segment of the arrow shows the
distance and the direction you move in a
given unit of time.
Because each segment is the same
length, you are moving the same distance
and in the same direction during each
interval of time.
Because both your speed and direction of
movement are constant, you are moving at
a constant velocity.
The velocity of an object changes if the
speed changes, the direction changes, or
both the speed and the direction change.
Velocity (cont.)
•  When either an object’s speed or
velocity changes, the object is
accelerating.
•  Acceleration is the measure of how
quickly the velocity of an object changes.
Velocity (cont.)
Can an object traveling at a
constant speed have a changing
velocity? Why or why not?
Calculating Acceleration
•  Average acceleration can be calculated
by using the following equation:
•  This equation refers only to a change in
speed, not direction.
•  The symbol vf represents final velocity,
vi represents initial velocity, and t
represents time it takes to make that
change in velocity.
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.
Using Graphs to Represent Motion
A displacement-time graph shows an
object’s speed and distance from the
reference point at any given time.
On a displacement-time graph, the x-axis
shows the time and the y-axis shows the
displacement from a reference point.
The line on a displacement-time
graph represents the average speed of
an object at a particular moment in time.
A speed-time graph shows an object’s
speed at any given time.
On a speed-time graph, the x-axis shows
the time, and the y-axis shows the speed
of an object.
The line on a speed-time graph shows
how the speed of an object changes as it
moves.
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.
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?
What are forces?
•  A push or pull on an object is a force.
•  A force has both size and direction.
•  Arrows are used to show the size and
direction of the force.
•  A force can change an object’s speed
and the direction in which the object is
moving.
Types of Forces
•  A contact force
is a push or a pull
applied by one
object to another
object that is
touching it.
•  Contact forces are
also called
mechanical forces.
Steve Cole/Getty Images
Types of Forces (cont.)
•  A force that pushes or pulls an object
without touching it is a
noncontact force.
•  Gravity is an
example of a
noncontact force.
Types of Forces (cont.)
What is the difference between
the way contact and noncontact
forces affect objects?
Friction
•  Friction is a contact force that resists
the sliding motion between two objects
that are touching.
•  The force of friction acts in the opposite
direction of the motion.
•  Rougher surfaces produce greater
friction than smooth surfaces.
Gravity
•  Gravity is a noncontact attractive force
that exists between all objects that have
mass.
•  Gravitational force depends on the mass
of the objects and the distance between
them.
Gravity (cont.)
•  Weight is a
measure of the
force of gravity
acting on an
object.
•  As two objects get
farther apart, the
gravitational force
between the
objects decreases.
•  As the mass
of one or both
objects increases,
the gravitational
force between
them increases.
•  Doubling the
mass of one of
the objects doubles
the force of attraction.
Gravity (cont.)
What factors affect the way
gravity acts on objects?
Combining Forces
•  When several forces act on an object,
the forces combine to act as a single
force.
•  The sum of the forces acting on an
object is called the net force.
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.)
When forces act in opposite directions,
you must include the direction of the
force when you add them.
Combining Forces (cont.)
When the net force on an object is 0 N, the
forces acting on it are balanced forces.
Combining Forces (cont.)
If the forces acting on an object are
balanced, the object’s motion does not
change.
•  When the net force acting on an object is
not 0, the forces acting on the object are
unbalanced forces.
•  Unbalanced forces cause objects to
change their motion, or accelerate.
Combining Forces (cont.)
How do balances and unbalanced
forces differ?
•  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.
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
•  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.)
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.
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.