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Lesson 1: Combining Forces
Lesson 2: Types of Force
Lesson 3: Unbalanced Forces
and Acceleration
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2.1 Combining Forces
force
contact force
noncontact force
net force
unbalanced forces
balanced forces
Newton’s first law of motion
Essential Question
How can forces
cause objects
to change their
motion?
2.1 Combining Forces
Force
• When more than one force acts on an
object, the combined effect is caused by
the sum of all applied forces.
• A push or a pull on an object is a force.
2.1 Combining Forces
Contact and Noncontact Forces
• A contact force is exerted only when two
objects are touching.
• A noncontact force is exerted when two
objects are not touching.
– EXAMPLE: Magnets exert a noncontact force
on each other.
2.1 Combining Forces
Force is a Vector
• Vectors have magnitude and direction.
• Force is a vector because it has a
direction and a magnitude.
• Force is measured in Newtons.
2.1 Combining Forces
Combining Forces
• When more than one force acts on an
object, the forces combine.
• The combination of all the forces acting on
an object is called the net force.
2.1 Combining Forces
Combining Forces in
the Same Direction
• Forces are vectors and require a reference
direction.
• When 2 or more forces are in the same
direction, you ADD the forces together for the
net force
2.1 Combining Forces
Combining Forces in
the Same Direction (cont.)
• If two people push the dresser to the right,
the forces are added together.
2.1 Combining Forces
Combining Forces in
Opposite Directions
• When forces in opposite directions
combine, the net force is in the direction
of the larger force.
• The size of the net force is the difference
between the two forces…you SUBTRACT
2.1 Combining Forces
Combining Forces in
Opposite Directions (cont.)
• Two people exerting a force in opposite
directions show how vectors combine to
create net force.
2.1 Combining Forces
Unbalanced and Balanced Forces
• If the net force on an object is not zero, it
is an unbalanced force.
• If the net force on an object does equal
zero, it is a balanced force.
2.1 Combining Forces
Balanced Forces
2.1 Combining Forces
Balanced Forces and Motion
• If two people push with the same force,
the door does not move.
• The forces have the same magnitude.
• The forces are in opposite directions.
• The net force on the door is zero and the
forces are balanced.
• The motion of the door does not change.
• Balanced forces mean the object does not
change motion
2.1 Combining Forces
Unbalanced Forces and Motion
• Unbalanced forces change motion.
• If two people push with different force, the
door moves.
• The forces have a different magnitude and
are in opposite directions.
• The net force on the door is not zero and
the forces are unbalanced.
• The door moves in the direction of the
larger force.
Newton’s First Law of Motion
• An object at rest, stays at rest
and an object in motion stays
in motion unless an
unbalanced force acts on it
2.1 Combining Forces
Newton’s First Law of Motion
• If the net force on an object is zero, the
velocity of the object does not change.
• If the net force is zero and the object is at
rest, it remains at rest.
• If the net force is zero and the object is
moving, it continues to move in a straight
line with constant speed.
Newton’s Laws
of Motion
2.1 Combining Forces
Inertia and Mass
• Mass is the amount of matter in an object.
• The size of the net force needed to cause
a certain change in motion depends on the
object’s mass.
• A bicycle is easier to push than a car
because it has less mass.
• Inertia is the ability of an object to resist a
change in motion
2.1 Combining Forces
Which of the following changes
occur when an unbalanced force
acts on an object?
A motion
B mass
C inertia
D weight
0%
0%
D
0%
C
0%
B
A
B
C
D
A
1.
2.
3.
4.
2.1 Combining Forces
What is the net force acting on a
door if one person pushes to the
right with a force of 2000 N and a
second person pushes with a force
of 1500 N to the right?
A 3500 N to the right
B 3500 N to the left
C 500 N to the left
0%
0%
D
D 500 N to the right
0%
C
0%
B
A
B
C
D
A
1.
2.
3.
4.
2.1 Combining Forces
0%
D
C
You are riding a bike. In which of the
following situations 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.
0%
0%
0%
D You pedal at
constant speed.
B
A
B
C
D
A
1.
2.
3.
4.
2.2 Types of Forces
gravity
compression force
law of universal
gravitation
normal force
weight
friction
elastic force
tension force
Essential Question
What are the
different types of
forces that can
act on an object?
2.2 Types of Forces
Gravity
• Many types of forces act on objects.
• Gravity is an attractive force between all
objects that have mass.
2.2 Types of Forces
Gravity (cont.)
• The ball does
not travel in a
straight line
because of the
unbalanced
force of gravity
acting on it.
2.2 Types of Forces
The Law of Universal Gravitation
• The Law of Universal Gravitation states
that all objects with mass are attracted to
each other.
• The magnitude of attraction depends on
the mass of each object and the distance
between them.
2.2 Types of Forces
The Law of Universal Gravitation (cont.)
• The gravitational force becomes stronger
as either or both objects increase in mass.
• The gravitational force becomes weaker as
the distance between the objects increases.
2.2 Types of Forces
Weight and Mass
• Mass is the amount of matter in an object
and does not change with location.
• Weight is the gravitational force on a
object and changes with location.
– Weight is a force and a vector.
– Weight changes with height above Earth.
2.2 Types of Forces
Friction
• Friction is a force that opposes the
movement between two surfaces in contact.
• A book pushed across a table slows down
because of friction.
• Friction is an unbalanced force acting on the
book to slow it down.
2.2 Types of Forces
Static Friction
• Static friction is the force between two
surfaces in contact that keeps them from
sliding when a force is applied.
• A force is applied to a heavy box, but the
box doesn’t move.
• The forces are balanced, the force pushing
the box equals the force of static friction
pushing in the opposite direction.
2.2 Types of Forces
Sliding Friction
• When the force pushing on the box is
larger than the maximum static friction
force, the box begins to slide.
• The frictional force that acts on a sliding
object that works to slow it down is called
sliding friction.
2.2 Types of Forces
Sliding Friction (cont.)
• The book pushed across the table slows
down because of sliding friction.
• Without sliding friction, the book would
continue moving without a force being
applied.
2.2 Types of Forces
Sliding Friction (cont.)
• Usually friction is present and an
unbalancing force must be applied to
keep an object moving.
• When friction is greatly reduced, objects
move with nearly constant velocity without an
applied force.
2.2 Types of Forces
Elastic Forces
• An elastic force occurs when a material is
stretched or compressed.
• A diving board exerts an upward elastic
force on the diver when it is bent
downward.
2.2 Types of Forces
Tension
• A tension force is a pulling force applied
by an object when it is stretched, such as a
rubber band.
2.2 Types of Forces
Compression
• A compression force is a squeezing force
applied to an object that can make an
object shrink.
• The size of the compression force exerted
by a material is equal to the size of the
force that compresses the material.
2.2 Types of Forces
Normal Forces
• A normal force is the force exerted by an
object that is perpendicular to the surface
of the object.
2.2 Types of Forces
Normal Forces (cont.)
• The normal force exerted by the table is an
elastic force.
• The weight of the
glass pushing
down on the table
is a compression
force.
2.2 Types of Forces
Normal Forces (cont.)
2.2 Types of Forces
Normal Forces (cont.)
2.2 Types of Forces
Forces in the Horizontal Direction
• Friction balances forces applied in a
horizontal direction.
• Friction equals the horizontal force on an
object that is not changing motion.
2.2 Types of Forces
Forces in the Vertical Direction
• Upward normal force balances the
downward force of gravity on an object that
is not moving vertically.
2.2 Types of Forces
A(n) ____ force is the force exerted
by an object that is perpendicular to
the surface of the object.
A compression
B elastic
C normal
D tension
0%
0%
D
0%
C
0%
B
A
B
C
D
A
1.
2.
3.
4.
2.2 Types of Forces
Which force causes a rolling ball
to slow down?
A sliding friction
B static friction
C normal force
D gravity
0%
0%
D
0%
C
0%
B
A
B
C
D
A
1.
2.
3.
4.
2.2 Types of Forces
The gravitational force between two
objects ____ as ____ increases.
A increases; distance
B decreases; mass
C increases; velocity
D decreases; distance
0%
0%
D
0%
C
0%
B
A
B
C
D
A
1.
2.
3.
4.
2.3 Unbalanced Forces
and Acceleration
centripetal force
Newton’s second law of motion
Newton’s third law of motion
Essential Question
How can
unbalanced
forces cause an
object to
accelerate?
2.3 Unbalanced Forces and Acceleration
Unbalanced Forces and Velocity
• Unbalanced forces cause accelerations.
• Without the upward normal force exerted by
the table, the ball accelerates downward
because of the unbalanced force of gravity.
2.3 Unbalanced Forces and Acceleration
Unbalanced Forces and Velocity (cont.)
• An unbalanced force applied to a moving
object in the same direction as the motion
speeds the object up.
2.3 Unbalanced Forces and Acceleration
Unbalanced Forces and Velocity (cont.)
• An unbalanced force applied to a moving
object in the opposite direction as the
motion slows the object down.
• Friction is applied in the opposite direction
and slows the sled down.
2.3 Unbalanced Forces and Acceleration
Unbalanced Forces and Velocity (cont.)
• Unbalanced forces can also change the
direction of motion.
• A thrown ball curves downward because of
the unbalanced force of gravity.
2.3 Unbalanced Forces and Acceleration
Unbalanced Forces and Velocity (cont.)
• A ball tied to a string and swung is a
horizontal circle has circular motion.
• The velocity of the ball
changes because the
direction of motion
changes.
• The string exerts an
unbalanced force on
the ball called
centripetal force.
2.3 Unbalanced Forces and Acceleration
Unbalanced Forces and Velocity (cont.)
• Centripetal force is the force perpendicular
to the velocity and pulls toward the center
of the circle.
• The string keeps the
ball moving in a circle.
• Unbalanced forces
cause an object to
speed up, slow down,
or change direction.
2.3 Unbalanced Forces and Acceleration
Newton’s Second Law of Motion
• The acceleration of an object equals the
net force divided by the object’s mass.
• Unbalanced forces cause an object to
accelerate.
What is Newton’s
second law of
motion?
Newton’s Second Law of Motion
• Force = Mass x Acceleration
2.3 Unbalanced Forces and Acceleration
Balanced Forces, Unbalanced Forces,
and Newton’s Second Law
• If the net force is equal to zero, the forces
are balanced.
• The acceleration is zero.
• The object’s motion does not change.
2.3 Unbalanced Forces and Acceleration
Balanced Forces, Unbalanced Forces,
and Newton’s Second Law (cont.)
• If the net force is not equal to zero, the
forces are unbalanced.
• The acceleration is not zero.
• The object changes motion.
2.3 Unbalanced Forces and Acceleration
Newton’s Second Law and
Planetary Motion
• According the Newton’s second law, there
must be an unbalanced force acting on
Earth and the other planets.
• The Sun’s gravity is the
centripetal force that
keeps Earth and the
planets moving in a
circular path around
the Sun.
2.3 Unbalanced Forces and Acceleration
Newton’s Third Law of Motion
• When one object exerts a force on another,
the second object exerts an equal force in
the opposite direction.
• The feet exert a force on the ground and
the ground exerts an equal force on the feet
of the jumper, causing the jumper to
accelerate upward.
2.3 Unbalanced Forces and Acceleration
Force Pairs
• The forces that two objects exert on each
other are called force pairs.
• The pairs are opposite in direction and
equal in magnitude
• The forces do not cancel out because they
act on different objects.
2.3 Unbalanced Forces and Acceleration
Action and Reaction
• One force in a force pair is called the action
force.
• The second force in a force pair is called
the reaction force.
• For every action force there is an opposite
and equal reaction force.
2.3 Unbalanced Forces and Acceleration
In what direction does the net force of
an object moving with constant speed
and in a circular motion occur?
A downward
B opposite to the
object’s motion
C toward the center
of the circle
0%
0%
0%
D
0%
C
D in the direction of
the object’s velocity
B
A
B
C
D
A
1.
2.
3.
4.
2.3 Unbalanced Forces and Acceleration
Acceleration depends on both the
force applied and ____.
A distance from
the object
B speed of the object
C mass of the object
0%
0%
0%
D
0%
C
D normal force of
the object
B
A
B
C
D
A
1.
2.
3.
4.
2.3 Unbalanced Forces and Acceleration
How does the motion of an object
traveling in a straight line at 5 m/s
change if a balanced force is applied?
A The motion does
not change.
B The object speeds up.
C The object slows down.
0%
0%
0%
D
0%
C
D The object travels in
a horizontal circle.
B
A
B
C
D
A
1.
2.
3.
4.