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Resources
Bellringers
Chapter Presentation
Transparencies
Standardized Test Prep
Visual Concepts
Math Skills
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Chapter 11
Forces
Table of Contents
Section 1 Laws of Motion
Section 2 Gravity
Section 3 Newton’s Third Law
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Chapter 11
Section 1 Laws of Motion
Objectives
• Identify the law that says that objects change their
motion only when a net force is applied.
• Relate the first law of motion to important
applications, such as seat belt safety issues.
• Calculate force, mass, and acceleration by using
Newton’s second law.
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Chapter 11
Section 1 Laws of Motion
Bellringer
In some cases, an applied force is balanced by an opposite force,
and there is no change in motion. In other cases, an applied force
is not balanced by an opposite force, and the result is acceleration
in the direction of the applied force. Look at the following
illustrations, and identify the forces and motion in each one.
(Illustrations are shown on the next slide.)
1. In one drawing, no motion is likely to occur. Which drawing is it?
2. In which diagram are the forces clearly balanced? How does
this relate to your answer to item 1? If more force is exerted
by the person, does the opposite force increase to match the
new force, stay the same or decrease?
3. Suppose there is enough friction in the wheels of the wagon in
diagram c. to balance the force with which the wagon is
pulled. How will this affect the motion of the wagon?
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Chapter 11
Section 1 Laws of Motion
Bellringer, continued
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Chapter 11
Section 1 Laws of Motion
Newton’s First Law
• Newton’s first law of motion states that an object at
rest remains at rest and an object in motion maintains
its velocity unless it experiences an unbalanced
force.
• Objects tend to maintain their state of motion.
• Inertia is the tendency of an object to resist being
moved or, if the object is moving, to resist a change
in speed or direction until an outside force acts on the
object.
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Chapter 11
Section 1 Laws of Motion
Newton’s First Law
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Chapter 11
Section 1 Laws of Motion
Newton’s First Law, continued
• Inertia is related to an object’s mass. Mass is a
measure of inertia.
• Seat belts and car seats provide protection.
• Because of inertia, you slide toward the side of a
car when the driver makes a sharp turn.
• When the car you are riding in comes to a stop,
your seat belt and the friction between you and the
seat stop your forward motion.
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Chapter 11
Section 1 Laws of Motion
Mass and Inertia
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Chapter 11
Section 1 Laws of Motion
Newton’s Second Law
• Newton’s second law of motion states that the
unbalanced force acting on an object equals the
object’s mass times its acceleration.
• Force equals mass times acceleration.
Force = mass  acceleration
F = ma
• Force is measured in newtons (N).
1 N = 1 kg  1 m/s2
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Chapter 11
Section 1 Laws of Motion
Newton’s Second Law
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Chapter 11
Section 1 Laws of Motion
Newton’s Second Law
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Chapter 11
Section 1 Laws of Motion
Math Skills
Newton’s Second Law Zookeepers lift a stretcher that
holds a sedated lion. The total mass of the lion and
stretcher is 175 kg, and the lion’s upward
acceleration is 0.657 m/s2. What is the unbalanced
force necessary to produce this acceleration of the
lion and the stretcher?
1. List the given and unknown values.
Given:
mass, m = 175 kg
acceleration, a = 0.657 m/s2
Unknown: force, F = ? N
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Chapter 11
Section 1 Laws of Motion
Math Skills, continued
2. Write the equation for Newton’s second law.
force = mass  acceleration
F = ma
3. Insert the known values into the equation, and
solve.
F = 175 kg  0.657 m/s2
F = 115 kg  m/s2 = 115 N
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Chapter 11
Section 1 Laws of Motion
Newton’s Second Law, continued
• Newton’s second law can also be stated as follows:
The acceleration of an object is proportional to the
net force on the object and inversely proportional
to the object’s mass.
force
acceleration 
mass
F
a
m
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Chapter 11
Section 2 Gravity
Objectives
• Explain that gravitational force becomes stronger as
the masses increase and rapidly becomes weaker as
the distance between the masses increases.
• Evaluate the concept that free-fall acceleration near
Earth’s surface is independent of the mass of the
falling object.
• Demonstrate mathematically how free-fall
acceleration relates to weight.
• Describe orbital motion as a combination of two
motions.
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Chapter 11
Section 2 Gravity
Bellringer
Recall that weight is defined as a measure of the gravitational force
exerted on an object. Use knowledge you have about gravity to
answer the questions in the following situations:
1. Elvis is a student whose mass is 70 kg. On Earth’s surface, Elvis
weighs about 690 N. Suppose Elvis could stand on the surface of
the following bodies in the solar system. In the blanks provided,
match Elvis’ weight with the letter of the appropriate body. (Note
that Earth has a mass of 6.0 x 1024 kg.)
Planet
Elvis’ weight
a. Jupiter (m = 1.9 x 1027 kg)
780 N _______
b. Venus (m = 4.9 x 1024 kg)
113 N _______
c. Neptune (m = 1.0 x 1026 kg)
260 N _______
d. Mercury (m = 3.3 x 1023 kg)
1800 N _______
e. Earth’s moon (m = 7.4 x 1022 kg)
620 N _______
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Chapter 11
Section 2 Gravity
Bellringer, continued
2. Suppose Elvis is in orbit around Venus at a distance twice as far
from the planet’s center as the surface of Venus is. Would you
expect his weight to be greater than, less than, or equal to his
weight on the surface of the planet?
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Chapter 11
Section 2 Gravity
Law of Universal Gravitation
• Sir Isaac Newton (1642–1727) generalized his
observations on gravity in a law now known as the
law of universal gravitation.
• Universal Gravitation Equation
F G
m1m2
d2
• m1 and m2 are the masses of the two objects
• d is the distance between the two objects
• G is a constant
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Chapter 11
Section 2 Gravity
Law of Universal Gravitation, continued
• All matter is affected by gravity.
• Two objects, whether large or small, always have
a gravitational force between them.
• When something is very large, like Earth, the force
is easy to detect.
• Gravitational force increases as mass increases.
• Gravitational force decreases as distance increases.
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Chapter 11
Section 2 Gravity
Law of Universal Gravitation
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Chapter 11
Section 2 Gravity
Law of Universal Gravitation
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Chapter 11
Section 2 Gravity
Free Fall and Weight
• Free fall is the motion of a body when only the force
of gravity is acting on the body.
• Free-fall acceleration near Earth’s surface is
constant.
• If we disregard air resistance, all objects near
Earth accelerate at 9.8 m/s2.
• Freefall acceleration is often abbreviated as the
letter g, so g = 9.8 m/s2.
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Chapter 11
Section 2 Gravity
Free Fall and Weight, continued
• Weight is equal to mass times free-fall acceleration.
weight = mass  free-fall acceleration
w = mg
• Weight is different from mass.
• Mass is a measure of the amount of matter in an
object.
• Weight is the gravitational force an object
experiences because of its mass.
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Chapter 11
Section 2 Gravity
Comparing Mass and Weight
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Chapter 11
Section 2 Gravity
Free Fall and Weight, continued
• Weight influences shape.
• Gravitational force influences the shape of living
things.
• Velocity is constant when air resistance balances
weight.
• The constant velocity of a falling object when the
force of air resistance is equal in magnitude and
opposite in direction to the force of gravity is called
the terminal velocity.
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Chapter 11
Section 2 Gravity
Terminal Velocity
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Chapter 11
Section 2 Gravity
Free Fall and Motion
• Orbiting objects are in
free fall.
• The moon stays in orbit
around Earth because
Earth’s gravitational force
provides a pull on the
moon.
• Two motions combine to
cause orbiting.
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Chapter 11
Section 2 Gravity
Two Motions Cause Orbiting
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Chapter 11
Section 2 Gravity
Free Fall
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Chapter 11
Section 2 Gravity
Projectile Motion and Gravity
• Projectile motion is the curved path an object
follows when thrown, launched, or otherwise
projected near the surface of Earth.
• Projectile motion applies to objects that are moving in
two dimensions under the influence of gravity.
• Projectile motion has two components—horizontal
and vertical. The two components are independent.
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Chapter 11
Section 2 Gravity
Projectile Motion
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Chapter 11
Section 2 Gravity
Projectile Motion and Gravity, continued
• Projectile motion has some horizontal motion.
• Horizontal motion is motion that is perpendicular
(90º) to Earth’s gravitational field.
• The horizontal velocity is constant.
• Projectile motion also has some vertical motion.
• The vertical motion is the same as downward freefall motion.
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Chapter 11
Section 2 Gravity
Projectile Motion
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Chapter 11
Section 3 Newton’s Third Law
Objectives
• Explain that when one object exerts a force on a
second object, the second object exerts a force equal
in size and opposite in direction on the first object.
• Show that all forces come in pairs commonly called
action and reaction pairs.
• Recognize that all moving objects have momentum.
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Chapter 11
Section 3 Newton’s Third Law
Bellringer
You have learned that forces
account for changes in the
motion of objects. Using what
you have learned, explained
what happens in the following
situation:
An ice skater holding a
basketball is standing on the
surface of a frozen pond. The
skater throws the ball forward.
At the same time, the skater
slides on the ice in the opposite
direction.
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Chapter 11
Section 3 Newton’s Third Law
Bellringer, continued
1. Is the force on the ball
greater than, less than, or equal
to the opposite force on the
skater?
2. Is the acceleration of the ball
greater than, less than, or equal
to the acceleration of the
skater? (Hint: Remember
Newton’s Second Law.)
3. Explain your answers.
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Chapter 11
Section 3 Newton’s Third Law
Action and Reaction Forces
• Newton’s third law of motion states that for every
action force, there is an equal and opposite reaction
force.
• Forces always occur in action-reaction pairs.
• Action-reaction force pairs are equal in size and
opposite in direction.
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Chapter 11
Section 3 Newton’s Third Law
Action and Reaction Forces, continued
• Force pairs do not act on the same object.
• When one object exerts an action force on a second
object, the second object exerts a reaction force on
the first object.
• Equal forces don’t always have equal effects.
• For example, the action force of Earth pulling on
an object and causing it to fall is much more
obvious than the equal and opposite reaction
force of the falling object pulling on Earth.
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Chapter 11
Section 3 Newton’s Third Law
Action and Reaction Forces
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Chapter 11
Section 3 Newton’s Third Law
Momentum
• Momentum is a quantity defined as the product of
the mass and velocity of an object.
momentum = mass  velocity
p = mv
• Moving objects have momentum.
• For a given velocity, the more mass an object has,
the greater its momentum is.
• Likewise, the faster an object is moving, the
greater its momentum is.
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Chapter 11
Section 3 Newton’s Third Law
Math Skills
Momentum Calculate the momentum of a 6.00 kg
bowling ball moving at 10.0 m/s down the alley
toward the pins.
1. List the given and unknown values.
Given:
mass, m = 6.00 kg
velocity, v = 10.0 m/s down the alley
Unknown: momentum, p = ? kg • m/s (and direction)
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Chapter 11
Section 3 Newton’s Third Law
Math Skills, continued
2. Write the equation for momentum.
momentum = mass x velocity
p = mv
3. Insert the known values into the equation, and
solve.
p = mv = 6.00 kg  10.0 m/s
p = 60.0 kg • m/s down the alley
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Chapter 11
Section 3 Newton’s Third Law
Momentum, continued
• Force is related to change in momentum.
• When you force an object to change its motion,
you force it to change its momentum.
• Momentum is conserved in collisions.
• The law of conservation of momentum states that
the total amount of momentum in an isolated
system is conserved.
• Conservation of momentum explains rocket
propulsion.
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Chapter 11
Section 3 Newton’s Third Law
Rocket Propulsion
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Chapter 11
Section 3 Newton’s Third Law
Momentum, continued
• Momentum is transferred.
• When a moving object hits a second object, some
or all of the momentum of the first object is
transferred to the second object.
• Momentum can be transferred in collisions, but the
total momentum before and after a collision is the
same.
• Action and reaction force pairs are everywhere.
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Chapter 11
Section 3 Newton’s Third Law
Momentum and Collisions
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Chapter 11
Section 3 Newton’s Third Law
Concept Mapping
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Chapter 11
Standardized Test Prep
Understanding Concepts
1. What is the net force on a 2.0 kg weight hanging
motionless on a string?
A. 0.0 N
B. 2.0 N
C. 9.8 N
D. 19.6 N
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Chapter 11
Standardized Test Prep
Understanding Concepts, continued
1. What is the net force on a 2.0 kg weight hanging
motionless on a string?
A. 0.0 N
B. 2.0 N
C. 9.8 N
D. 19.6 N
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Chapter 11
Standardized Test Prep
Understanding Concepts, continued
2. What is the source of the force that causes a jet
airplane to accelerate forward?
F. gravitational pull
G. air pressure on the wings
H. exhaust gases pushing against the engine
I. exhaust gases pushing against the atmosphere
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Chapter 11
Standardized Test Prep
Understanding Concepts, continued
2. What is the source of the force that causes a jet
airplane to accelerate forward?
F. gravitational pull
G. air pressure on the wings
H. exhaust gases pushing against the engine
I. exhaust gases pushing against the atmosphere
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Chapter 11
Standardized Test Prep
Understanding Concepts, continued
3. Why does a skydiver not accelerate downward after
reaching terminal velocity?
A. The force of gravity is inactive on the skydiver at
terminal velocity.
B. Air resistance exceeds the force of gravity.
C. Air resistance balances the force of gravity.
D. The force of gravity decreases as the skydiver
descends.
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Chapter 11
Standardized Test Prep
Understanding Concepts, continued
3. Why does a skydiver not accelerate downward after
reaching terminal velocity?
A. The force of gravity is inactive on the skydiver at
terminal velocity.
B. Air resistance exceeds the force of gravity.
C. Air resistance balances the force of gravity.
D. The force of gravity decreases as the skydiver
descends.
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Chapter 11
Standardized Test Prep
Understanding Concepts, continued
4. The ancient Greek scientist, Aristotle, claimed that
the speed of a falling object depends on its weight.
But you can disprove his hypothesis by dropping a
pen and a baseball simultaneously and observing
when they hit the floor.
Why do falling objects not act as Aristotle thought
they would?
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Chapter 11
Standardized Test Prep
Understanding Concepts, continued
4. The ancient Greek scientist, Aristotle, claimed that
the speed of a falling object depends on its weight.
But you can disprove his hypothesis by dropping a
pen and a baseball simultaneously and observing
when they hit the floor.
Why do falling objects not act as Aristotle thought
they would?
Answer:The acceleration due to gravity depends on the
total masses of the object and Earth. Because Earth
is so much larger than either of the objects, the
acceleration depends on the mass of Earth and is
the same for both objects.
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Chapter 11
Standardized Test Prep
Understanding Concepts, continued
5. Analyze why you would weigh less on the surface of
Mars, even though your body remains exactly the
same size and shape.
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Chapter 11
Standardized Test Prep
Understanding Concepts, continued
5. Analyze why you would weigh less on the surface of
Mars, even though your body remains exactly the
same size and shape.
Answer: Weight is a measure of the force of gravity on
an object. The mass of your body stays the same,
but the force on it is less on Mars.
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Chapter 11
Standardized Test Prep
Reading Skills
The United States Air Force trains astronauts in a
large jet airplane, which is known as the “Vomit
Comet” because many people get airsick during its
flight. The plane accelerates upward and then falls
back toward Earth, in the form of an arc. At the peak
of its flight, the passengers seem to float inside the
plane, and objects around them appear to be
unaffected by gravity for about 20 seconds during
each arc.
6. Describe the forces that are acting on the passengers
as the plane begins its acceleration upward towards
the top of the arc.
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Chapter 11
Standardized Test Prep
Reading Skills, continued
[See previous slide for reading passage.]
6. Describe the forces that are acting on the
passengers as the plane begins its acceleration
upward towards the top of the arc.
Answer: The forces acting on the passengers are
gravitational pull of Earth and an additional
downward force against the airplane, which is
moving upward.
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Chapter 11
Standardized Test Prep
Interpreting Graphics
7. When a ball is falling toward
Earth, what is the reaction
force to the pull of gravity
on the ball?
F. air pressure pushing up
on the ball
G. force of the ground
against the ball
H. upward pull of the ball
on Earth
I. pull of gravity on the
ball toward Earth
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Chapter 11
Standardized Test Prep
Interpreting Graphics, continued
7. When a ball is falling toward
Earth, what is the reaction
force to the pull of gravity
on the ball?
F. air pressure pushing up
on the ball
G. force of the ground
against the ball
H. upward pull of the ball
on Earth
I. pull of gravity on the
ball toward Earth
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