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Chapter 10
Motion
Table of Contents
Section 1 Measuring Motion
Section 2 Acceleration
Section 3 Motion and Force
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Chapter 10
Section 1 Measuring Motion
Objectives
• Explain the relationship between motion and a frame
of reference.
• Relate speed to distance and time.
• Distinguish between speed and velocity.
• Solve problems related to time, distance, displacement, speed, and velocity.
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Chapter 10
Section 1 Measuring Motion
Bellringer
Everybody knows what motion is, but how do you measure it?
•
•
•
One way is to measure distance, how far something goes during a motion.
Another is to measure time, how long a motion takes to occur.
A third way is to measure speed, how fast something is moving.
Each of the devices shown below can be used to measure some
aspect of motion.
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Chapter 10
Section 1 Measuring Motion
Bellringer, continued
1. For each of the devices above, indicate whether it measures
distance, time, or speed.
2. For each of the devices above, indicate which of the following
units are possible for a measurement: meters (m), seconds (s),
or meters per second (m/s).
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Chapter 10
Section 1 Measuring Motion
Observing Motion
• Motion is an object’s change in position relative to a
reference point.
• Displacement is the change in the position of an
object.
• Distance measures the path taken.
• Displacement is the change of an object’s position.
Displacement must always indicate direction.
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Chapter 10
Section 1 Measuring Motion
Distance vs. Displacement
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Chapter 10
Section 1 Measuring Motion
Speed and Velocity
• Speed is the distance traveled divided by the time
interval during which the motion occurred. Speed
describes how fast an object moves.
• Speed measurements involve distance and time.
• The SI units for speed are meters per second
(m/s).
• When an object covers equal distances in equal
amounts of time, it is moving at a constant speed.
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Chapter 10
Section 1 Measuring Motion
Speed and Velocity, continued
• Speed can be studied with
graphs and equations.
• Speed can be determined
from a distance-time graph.
When an object’s motion is
graphed by plotting
distance on the y-axis and
time on the x-axis, the
slope of the graph is
speed.
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Chapter 10
Section 1 Measuring Motion
Speed and Velocity, continued
• Average speed is calculated as distance divided by
time.
Equation for Average Speed
distance
speed =
time
d
v=
t
• Instantaneous speed is the speed at a given point in
time.
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Chapter 10
Section 1 Measuring Motion
Math Skills
Velocity Metal stakes are sometimes placed in glaciers
to help measure a glacier’s movement. For several
days in 1936, Alaska’s Black Rapids glacier surged
as swiftly as 89 meters per day down the valley. Find
the glacier’s velocity in m/s. Remember to include
direction.
1. List the given and the unknown values.
Given:
time, t = 1 day
displacement, d = 89 m down the valley
Unknown: velocity, v = ? (m/s and direction)
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Chapter 10
Section 1 Measuring Motion
Math Skills, continued
2. Perform any necessary conversions.
To find the velocity in meters per second, the
value for time must be in seconds.
60 min 60 s
t = 1 day = 24 h  1 h  1 min
t = 86 400 s = 8.64  104 s
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Chapter 10
Section 1 Measuring Motion
Math Skills, continued
3. Write the equation for speed.
displacement
d
speed =
=
time
t
4. Insert the known values into the equation, and
solve.
89 m
d
v= t =
(For velocity, include direction.)
8.64  104 s
v = 1.0  10–3 m/s down the valley
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Chapter 10
Section 1 Measuring Motion
Speed and Velocity, continued
• Velocity is the speed of an object in a particular
direction.
• Velocity describes both the speed and the direction.
• Combine velocities to determine the resultant
velocity.
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Chapter 10
Section 2 Acceleration
Objectives
• Describe the concept of acceleration as a change in
velocity.
• Explain why circular motion is continuous
acceleration even when the speed does not change.
• Calculate acceleration as the rate at which velocity
changes.
• Graph acceleration on a velocity-time graph.
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Chapter 10
Section 2 Acceleration
Bellringer
Which of the following examples shows a change in
velocity? Remember a change in velocity can be
either a change in speed or a change in the direction
of motion. Briefly explain your answer.
a. a car coming to a stop at a stop sign
b. a book sitting on a desk
c. a yo-yo in motion
d. a bicyclist making a left-hand turn at exactly 15 km/h
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Chapter 10
Section 2 Acceleration
Bellringer, continued
In the picture shown above, the student pulls on the box with the
rope. If the box is originally not moving, will its velocity increase
or stay the same? In which direction (if any) will the velocity
be after the student pulls on the box with the rope?
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Chapter 10
Section 2 Acceleration
Acceleration and Motion
• Acceleration is the rate at which velocity changes
over time.
• An object accelerates if its speed, direction, or both
change.
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Chapter 10
Section 2 Acceleration
Acceleration and Motion, continued
• Acceleration can be a change in speed.
• Acceleration can be a change in direction.
• Uniform circular motion is constant acceleration.
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Chapter 10
Section 2 Acceleration
Calculating Acceleration
• Acceleration is the rate at which velocity changes.
Acceleration Equation (for straight-line motion)
∆v
final velocity – initial velocity
acceleration =
a= t
time
• In SI units, acceleration is measured in meters per
second per second (m/s/s) or m/s2.
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Chapter 10
Section 2 Acceleration
Math Skills
Acceleration A flowerpot falls off a second-story
windowsill. The flowerpot starts from rest and hits
the sidewalk 1.5 s later with a velocity of 14.7 m/s.
Find the average acceleration of the flowerpot.
1. List the given and the unknown values.
Given:
time, t = 1.5 s
initial velocity, vi = 0 m/s
final velocity, vf = 14.7 m/s down
Unknown: acceleration, a = ? (m/s2 and direction)
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Chapter 10
Section 2 Acceleration
Math Skills, continued
2. Write the equation for acceleration.
final velocity – initial velocity vf – vi
acceleration =
= t
time
3. Insert the known values into the equation, and
solve.
vf – vi 14.7 m/s – 0 m/s
a= t =
1.5 s
14.7 m/s
a = 1.5 s = 9.8 m/s2 down
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Chapter 10
Section 2 Acceleration
Calculating Acceleration, continued
Acceleration can be
determined from a
velocity-time graph.
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Chapter 10
Section 3 Motion and Force
Objectives
• Explain the effects of unbalanced forces on the
motion of objects.
• Compare and contrast static and kinetic friction.
• Describe how friction may be either harmful or
helpful.
• Identify ways in which friction can be reduced or
increased.
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Chapter 10
Section 3 Motion and Force
Bellringer
The concept of force explains many occurrences in our everyday
lives. From your own experience, state what will happen in the
following situations:
1. A marble is placed at the top of a smooth ramp. What happens to
the marble? What force causes this?
2. A marble is rolling around in the back of a small toy wagon as the
wagon is pulled along the sidewalk. When the wagon is stopped
suddenly by a rock under one of the wheels, the marble rolls
towards the front of the wagon. Why does the marble keep going
when the wagon stops? (Hint: Consider what it takes to change
the velocity of the wagon and the marble.)
3. If you dropped a flat uncrumpled sheet of notebook paper and a
similar piece of notebook paper that was crushed into a ball from
the same height, which would reach the floor first? Why are
the forces on these two pieces of paper different?
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Chapter 10
Section 3 Motion and Force
Balanced and Unbalanced Forces
• Force is an action exerted on a body in order to
change the body’s state of rest or motion. Force has
magnitude and direction.
• The net force is the combination of all forces
acting on an object.
• Objects subjected to balanced forces either do not
move or move at constant velocity.
• An unbalanced force must be present to cause any
change in an object’s state of motion or rest.
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Chapter 10
Section 3 Motion and Force
The Force of Friction
• Friction is a force that opposes motion between two
surfaces that are in contact.
• Friction opposes the applied force.
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Chapter 10
Section 3 Motion and Force
The Force of Friction, continued
• Static friction resists the initiation of sliding motion
between two surfaces that are in contact and at rest.
• Kinetic friction opposes the movement of two
surfaces that are in contact and are sliding over each
other.
• Static friction is greater than kinetic friction.
• There are many different types of kinetic friction, such
as sliding friction and rolling friction.
• Fluid friction, such as air resistance, also opposes
motion.
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Chapter 10
Section 3 Motion and Force
Frictional Forces and Acceleration
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Chapter 10
Section 3 Motion and Force
Friction and Motion
• Friction can be helpful or harmful.
• Friction is necessary to roll a vehicle or hold an
object.
• However, friction can also cause excessive
heating or wear of moving parts.
• Harmful friction can be reduced.
• Helpful friction can be increased.
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Chapter 10
Standardized Test Prep
Understanding Concepts
1. A bicyclist traveling at 10 m/s applies her brakes,
reducing her velocity to 5 m/s. If it takes 2 seconds
to slow the bike, what is the acceleration during that
period?
A.
B.
C.
D.
–5 m/s2
–2.5 m/s2
0 m/s2
2.5 m/s2
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Chapter 10
Standardized Test Prep
Understanding Concepts, continued
1. A bicyclist traveling at 10 m/s applies her brakes,
reducing her velocity to 5 m/s. If it takes 2 seconds
to slow the bike, what is the acceleration during that
period?
A.
B.
C.
D.
–5 m/s2
–2.5 m/s2
0 m/s2
2.5 m/s2
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Chapter 10
Standardized Test Prep
Understanding Concepts, continued
2. What happens to a moving object when the forces
acting on it are exactly balanced?
F. The object gradually slows and stops moving due
to friction.
G. The object moves at a constant speed, neither
speeding up nor slowing down.
H. The object accelerates because there is no
friction to oppose the forces acting on it.
I. The object accelerates because the force of
gravity is stronger than friction.
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Understanding Concepts, continued
2. What happens to a moving object when the forces
acting on it are exactly balanced?
F. The object gradually slows and stops moving due
to friction.
G. The object moves at a constant speed, neither
speeding up nor slowing down.
H. The object accelerates because there is no
friction to oppose the forces acting on it.
I. The object accelerates because the force of
gravity is stronger than friction.
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Understanding Concepts, continued
3. For which of the following is the velocity constant?
A. a baseball traveling away from a bat after a hit
B. a bicyclist on an oval track moving at a constant
speed of 15 km/h
C. a helicopter hovering motionless above a fixed
point on the ground
D. a canoe being carried down a winding river by a
steady current
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Understanding Concepts, continued
3. For which of the following is the velocity constant?
A. a baseball traveling away from a bat after a hit
B. a bicyclist on an oval track moving at a constant
speed of 15 km/h
C. a helicopter hovering motionless above a fixed
point on the ground
D. a canoe being carried down a winding river by a
steady current
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Understanding Concepts, continued
4. When describing the motion of an object, why do
you need a reference point?
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Understanding Concepts, continued
4. When describing the motion of an object, why do
you need a reference point?
To describe the motion of an object, you need to
compare the object’s position to a reference point to
determine how the position is changing.
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Understanding Concepts, continued
5. A jet airplane must use a substantial amount of fuel
to keep flying at a constant velocity. What two forces
would change the velocity of the plane if the engines
were shut off?
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Understanding Concepts, continued
5. A jet airplane must use a substantial amount of fuel
to keep flying at a constant velocity. What two forces
would change the velocity of the plane if the engines
were shut off?
Air resistance and the force due to gravity would
change the velocity of the plane.
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Chapter 10
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Reading Skills
Read the passage below. Then, answer question 6.
Some boats used in calm waters actually float on a
cushion of air. A fan blows air below the hull of the
craft, while another fan pushes air backward,
propelling the boat forward. If the downward fan is
turned off, the boat stops moving. The rear-facing
fan cannot cause the boat to move until the other fan
is turned on again.
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Reading Skills, continued
6. Demonstrate why the rear-facing fan can accelerate
the boat when it is on a cushion of air, but not when
it is in contact with the surface of the water.
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Reading Skills, continued
6. Demonstrate why the rear-facing fan can accelerate
the boat when it is on a cushion of air, but not when
it is in contact with the surface of the water.
The boat experiences less friction when it is on a
cushion of air than it does when it is in contact with
the water. The rear-facing fan provides enough force
to overcome only the lesser friction.
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Chapter 10
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Interpreting Graphics
Base you answers to questions 7 and 8 on the graph
below, which shows distance (m) versus time (s).
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Interpreting Graphics, continued
7. What is the average speed of the runner whose
motion is plotted on the graph?
F.
G.
H.
I.
1.0 m/s
1.8 m/s
2.0 m/s
4.5 m/s
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Interpreting Graphics, continued
7. What is the average speed of the runner whose
motion is plotted on the graph?
F.
G.
H.
I.
1.0 m/s
1.8 m/s
2.0 m/s
4.5 m/s
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Interpreting Graphics, continued
8. During which interval is the runner’s average speed
the greatest?
A.
B.
C.
D.
0 s to 5 s
5 s to 10 s
15 s to 20 s
20 s to 25 s
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Interpreting Graphics, continued
8. During which interval is the runner’s average speed
the greatest?
A.
B.
C.
D.
0 s to 5 s
5 s to 10 s
15 s to 20 s
20 s to 25 s
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