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Chapter 12
Matter in Motion
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Section 1 Measuring Motion
Section 2 What Is a Force?
Section 3 Friction: A Force That Opposes Motion
Concept Map
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Chapter 12
Section 1 Measuring Motion
Bellringer
Describe the location of a favorite place, using a
reference point and reference directions. For
example, “The library is at the corner of Madison
Street and Monroe Street. It is two blocks west of the
school and four blocks north of the bus stop.”
Write your response in your Science Journal.
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Chapter 12
Section 1 Measuring Motion
What You Will Learn
• Properties used to describe the motion of an object
include a reference point, direction, speed, velocity,
and acceleration.
• Average speed can be calculated by dividing total
distance by total time.
• A change in velocity is due to a change in speed,
direction, or both.
• Speed and acceleration can be represented on
graphs.
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Chapter 12
Section 1 Measuring Motion
Motion and Reference Points
• An object is in motion if it changes position over time
in relation to a reference point.
• A reference point is any object that appears to stay in
place.
• Features on Earth’s surface are often used as
standard reference points.
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Chapter 12
Section 1 Measuring Motion
Motion and Reference Points, continued
• Reference directions—such as north, south, east,
west, up, or down—can be used to describe the
direction of an object’s motion.
• A grid can be used to describe motion in a twodimensional system.
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Chapter 12
Section 1 Measuring Motion
Average Speed
• The speed of an object is the rate at which the object
moves.
• The speed of an object is rarely constant. So, it is
useful to find the average speed of an object.
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Chapter 12
Section 1 Measuring Motion
Average Speed, continued
• Average speed is the total distance traveled divided
by the total time taken.
• A graph of position versus time can show an object’s
speed. On a graph of position versus time, the slope
of the line is equal to the speed of the object.
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Chapter 12
Section 1 Measuring Motion
Velocity: Direction Matters
• Velocity is the speed of an object in a particular
direction.
• Velocity and speed do not have the same meaning.
• Velocity must include a direction.
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Chapter 12
Section 1 Measuring Motion
Velocity: Direction Matters, continued
• The velocity of an object is constant only if the speed
and direction of the object do not change. So,
constant velocity is always motion along a straight
line.
• The velocity of an object changes if the object’s
speed, direction, or both change.
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Chapter 12
Matter in Motion
Velocity
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Chapter 12
Section 1 Measuring Motion
Acceleration
• Acceleration is the rate at which velocity changes
over time.
• An object can accelerate by changing speed,
direction, or both.
• The units for acceleration are the units for velocity
divided by a unit for time, usually meters per second
per second, or (m/s)/s, which is also written as m/s2.
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Chapter 12
Matter in Motion
Acceleration
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Chapter 12
Section 1 Measuring Motion
Acceleration, continued
• Centripetal acceleration occurs when an object
moves at a constant speed in circular motion.
• A graph of speed versus time can show positive
acceleration, no acceleration, and negative
acceleration.
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Chapter 12
Matter in Motion
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Chapter 12
Section 2 What Is a Force?
Bellringer
A force is a push or a pull on an object. Imagine that
you are trying to push a box up a ramp. Describe as
many forces (pushes and pulls) as you can in that
situation. Be sure to include the direction of each
force (push or pull) in your description.
Write your answers in your Science Journal.
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Chapter 12
Section 2 What Is a Force?
What You Will Learn
• A force is a push or a pull that acts on an object.
Forces have magnitude and direction.
• Net force is the combined effect of two or more forces
acting on an object.
• Balanced forces do not cause changes in motion.
• Unbalanced forces cause changes in an object’s
velocity.
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Chapter 12
Section 2 What Is a Force?
Forces Acting on Objects
• A force is a push or a pull exerted on an object in
order to change the motion of the object.
• All forces have two properties: direction and
magnitude, or size.
• Forces can cause acceleration.
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Chapter 12
Section 2 What Is a Force?
Forces Acting on Objects, continued
• Forces always act on objects. For any push to occur,
something has to receive the push.
• A newton (N) is the SI unit used to describe the
magnitude, or size, of a force.
• A force can act on an object without causing the
object to move.
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Chapter 12
Matter in Motion
Force
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Chapter 12
Section 2 What Is a Force?
Combined Effect of Forces
• The net force is the combination of all the forces
acting on an object.
• When forces act in the same direction, you add the
forces to determine the net force.
• When two forces act in opposite directions, you
subtract the smaller force from the larger force to
determine the net force.
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Chapter 12
Matter in Motion
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Chapter 12
Section 2 What Is a Force?
Balanced Forces: No Change in Motion
• When the forces on an object produce a net force of
0 N, the forces are balanced.
• Balanced forces do not cause a change in the motion
of an object.
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Chapter 12
Section 2 What Is a Force?
Balanced Forces: No Change in Motion,
continued
• Objects that are not moving and have balanced
forces acting on them are called static objects.
• Tension and compression are two forces that often
act on static objects.
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Chapter 12
Section 2 What Is a Force?
Balanced Forces: No Change in Motion,
continued
• Tension is a force that is exerted when matter is
pulled or stretched.
• Compression is a force that is exerted when matter is
pushed or squeezed.
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Chapter 12
Section 2 What Is a Force?
Unbalanced Forces: Velocity Changes
• When the net force on an object is not 0 N, the forces
on the object are unbalanced.
• Unbalanced forces cause a change in the velocity of
an object.
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Chapter 12
Section 2 What Is a Force?
Unbalanced Forces: Velocity Changes,
continued
• Unbalanced forces are needed to cause static
objects to start moving.
• Objects do not always move in the direction of the
unbalanced force.
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Chapter 12
Section 3 Friction: A Force That
Opposes Motion
Bellringer
Imagine that you are trying to push a heavy desk across
the floor. There are two opposing forces on the desk:
the force you apply, and the force of friction between
the desk and the floor. What will happen if the force
of friction is the same magnitude as the force that you
apply? What will happen if the force that you apply is
greater than the force of friction?
Write your answers in your Science Journal.
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Chapter 12
Section 3 Friction: A Force That
Opposes Motion
What You Will Learn
• The magnitude of the force of friction can vary.
• Kinetic friction is a force that, when unbalanced, can
change the velocity of a moving object.
• Static friction balances an applied force and can
prevent motion.
• Friction can be both harmful and helpful.
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Chapter 12
Section 3 Friction: A Force That
Opposes Motion
The Source of Friction
• When the hills and valleys of one surface stick to the
hills and valleys of another surface, friction is created.
• Friction is a force that opposes motion between two
surfaces that are touching.
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Chapter 12
Section 3 Friction: A Force That
Opposes Motion
The Source of Friction, continued
• As the force pushing surfaces together increases, the
friction between the surfaces increases.
• Friction is usually greater between materials that
have rough surfaces than it is between materials that
have smooth surfaces.
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Chapter 12
Matter in Motion
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Chapter 12
Section 3 Friction: A Force That
Opposes Motion
Types of Friction
• There are two types of friction: kinetic friction and
static friction.
• Kinetic friction is friction between moving surfaces.
• The force of sliding kinetic friction is greater than the
force of rolling kinetic friction.
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Chapter 12
Section 3 Friction: A Force That
Opposes Motion
Types of Friction, continued
• Static friction can balance an applied force and can
prevent changes in motion.
• Some static objects do not move because the force
of static friction balances the other forces acting on
the object.
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Chapter 12
Matter in Motion
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Chapter 12
Section 3 Friction: A Force That
Opposes Motion
Friction: Harmful and Helpful
• Because friction can be both harmful and helpful,
sometimes it should be decreased and sometimes it
should be increased.
• Lubricants are substances that are put on surfaces to
decrease friction between the surfaces. Some
examples of lubricants include motor oil, wax, and
grease.
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Chapter 12
Section 3 Friction: A Force That
Opposes Motion
Friction: Harmful and Helpful, continued
• Harmful friction can be decreased by using lubricants
or by smoothing the surfaces that rub against each
other.
• Helpful friction can be increased by making the
surfaces that rub against each other rougher or by
increasing the force pushing surfaces together.
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Chapter 12
Matter in Motion
Concept Map
Use the terms below to complete the concept map
on the next slide.
speed
time
velocity
motion
acceleration
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Chapter 12
Matter in Motion
Concept Map
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Chapter 12
Matter in Motion
Concept Map
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