motion in one dimension - university of north floridan00006757/physicslectures/knight...
TRANSCRIPT
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© 2010 Pearson Education, Inc.
PowerPoint® Lectures for
College Physics: A Strategic Approach, Second Edition
Chapter 2
Motion in One
Dimension
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© 2010 Pearson Education, Inc. Slide 2-2
2 Motion in One Dimension
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© 2010 Pearson Education, Inc. Slide 2-3
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© 2010 Pearson Education, Inc. Slide 2-4
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© 2010 Pearson Education, Inc. Slide 2-5
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© 2010 Pearson Education, Inc. Slide 2-6
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© 2010 Pearson Education, Inc.
Reading Quiz
1. The slope at a point on a position-versus-time graph of an
object is
A. the object’s speed at that point.
B. the object’s average velocity at that point.
C. the object’s instantaneous velocity at that point.
D. the object’s acceleration at that point.
E. the distance traveled by the object to that point.
Slide 2-7
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Answer
1. The slope at a point on a position-versus-time graph of an
object is
A. the object’s speed at that point.
B. the object’s average velocity at that point.
C. the object’s instantaneous velocity at that point.
D. the object’s acceleration at that point.
E. the distance traveled by the object to that point.
Slide 2-8
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© 2010 Pearson Education, Inc.
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© 2010 Pearson Education, Inc.
Reading Quiz
2. The area under a velocity-versus-time graph of an object is
A. the object’s speed at that point.
B. the object’s acceleration at that point.
C. the distance traveled by the object.
D. the displacement of the object.
E. This topic was not covered in this chapter.
Slide 2-9
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Answer
2. The area under a velocity-versus-time graph of an object is
A. the object’s speed at that point.
B. the object’s acceleration at that point.
C. the distance traveled by the object.
D. the displacement of the object.
E. This topic was not covered in this chapter.
Slide 2-10
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© 2010 Pearson Education, Inc.
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Reading Quiz
3. A 1-pound ball and a 100-pound ball are dropped from a height
of 10 feet at the same time. In the absence of air resistance
A. the 1-pound ball hits the ground first.
B. the 100-pound ball hits the ground first.
C. the two balls hit the ground at the same time.
D. There’s not enough information to determine which ball
wins the race.
Slide 2-11
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Answer
3. A 1-pound ball and a 100-pound ball are dropped from a height
of 10 feet at the same time. In the absence of air resistance
A. the 1-pound ball hits the ground first.
B. the 100-pound ball hits the ground first.
C. the two balls hit the ground at the same time.
D. There’s not enough information to determine which ball
wins the race.
Slide 2-12
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Representations
Motion diagram (student walking to school)
Table of data Graph
Slide 2-13
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Example Problem
A car moves along a straight stretch of road. The graph below
shows the car’s position as a function of time.
At what point (or points) do the following conditions apply?
• The displacement is zero.
• The speed is zero.
• The speed is increasing.
• The speed is decreasing.
Slide 2-14
D
B, E
C
A
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© 2010 Pearson Education, Inc. Slide 2-16
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Here is a motion diagram of a car moving along a straight stretch
of road:
Which of the following velocity-versus-time graphs matches this
motion diagram?
Checking Understanding
A. B. C. D.
Slide 2-17
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Here is a motion diagram of a car moving along a straight stretch
of road:
Which of the following velocity-versus-time graphs matches this
motion diagram?
Answer
A. B. C. D.
Slide 2-18
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Checking Understanding
A graph of position versus time for a
basketball player moving down the
court appears like so:
Which of the following velocity graphs matches the above
position graph?
A. B. C. D.
Slide 2-19
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A graph of position versus time for a
basketball player moving down the
court appears like so:
Which of the following velocity graphs matches the above
position graph?
Answer
A. B. C. D.
Slide 2-20
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A graph of velocity versus time for a
hockey puck shot into a goal appears
like so:
Which of the following position graphs matches the above
velocity graph?
Checking Understanding
A. B. C. D.
Slide 2-21
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A graph of velocity versus time for a
hockey puck shot into a goal appears
like so:
Which of the following position graphs matches the above
velocity graph?
Answer
A. B. C. D.
Slide 2-22
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© 2010 Pearson Education, Inc. Slide 2-23
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© 2010 Pearson Education, Inc. Slide 2-24
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Example Problem
A soccer player is 15 m from her opponent’s goal. She kicks the
ball hard; after 0.50 s, it flies past a defender who stands 5 m
away, and continues toward the goal. How much time does the
goalie have to move into position to block the kick from the
moment the ball leaves the kicker’s foot?
Slide 2-25
f i
x x v t
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Acceleration
Acceleration is:
• The rate of change of
velocity
• The slope of a velocity-
versus-time graph
Slide 2-26
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© 2010 Pearson Education, Inc. Slide 2-26
*x x
x x xf i
v a t
v v a t
x x xf iv v a t
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These four motion diagrams show the motion of a particle along
the x-axis. Rank these motion diagrams by the magnitude of the
acceleration. There may be ties.
Checking Understanding
A. B A D C
B. B A C D
C. A B C D
D. B D A C
Slide 2-27
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© 2010 Pearson Education, Inc.
These four motion diagrams show the motion of a particle along
the x-axis. Rank these motion diagrams by the magnitude of the
acceleration. There may be ties.
Answer
A. B A D C
B. B A C D
C. A B C D
D. B D A C
Slide 2-28
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These four motion diagrams show the motion of a particle along
the x-axis. Which motion diagrams correspond to a positive
acceleration? Which motion diagrams correspond to a negative
acceleration?
Checking Understanding
A. C and D
B. B and D
C. A, B, C, and D
D. A and C
Slide 2-29
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These four motion diagrams show the motion of a particle along
the x-axis. Which motion diagrams correspond to a positive
acceleration? Which motion diagrams correspond to a negative
acceleration?
Answer
A. C and D
B. B and D
C. A, B, C, and D
D. A and C
Slide 2-30
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These six motion diagrams show the motion of a particle along the
x-axis. Rank the accelerations corresponding to these motion
diagrams, from most positive to most negative. There may be ties.
Checking Understanding
A. A D E F B C
B. B C E F A D
C. A B C D E F
D. A B C D E F
Slide 2-31
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These six motion diagrams show the motion of a particle along the
x-axis. Rank the accelerations corresponding to these motion
diagrams, from most positive to most negative. There may be ties.
Checking Understanding
A. A D E F B C
B. B C E F A D
C. A B C D E F
D. A B C D E F
Slide 2-32
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A ball moving to the right traverses the ramp shown below. Sketch a
graph of the velocity versus time, and directly below it, using the
same scale for the time axis, sketch a graph of the acceleration
versus time.
Example Problem
Slide 2-33
Unspoken assumption…
…gravity
a g
-g
v
0v
t
t
a
Note that the velocity and acceleration in the graphs are along the path of the ball (see above),not vertically or horizontally.
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h
hosin
h
hacos
a
o
h
htan
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h
ho1sin
h
ha1cos
a
o
h
h1tan
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Tennis balls are tested by measuring their bounce when dropped
from a height of approximately 2.5 m. What is the final speed of a
ball dropped from this height?
Example Problem
Slide 2-34
2
2.5 m, 0 2.5 m
9.8 m/s?
i f
f
y y y
a gv
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Tennis balls are tested by measuring their bounce when dropped
from a height of approximately 2.5 m. What is the final speed of a
ball dropped from this height?
Example Problem
Slide 2-34
2
2 2
2.5 m, 0 2.5 m
9.8 m/s?
Use this kinematics equation:
2
i f
f
y yf i
y y y
a gv
v v a y
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Tennis balls are tested by measuring their bounce when dropped
from a height of approximately 2.5 m. What is the final speed of a
ball dropped from this height?
Example Problem
Slide 2-34
2
2 2
22
2.5 m, 0 2.5 m
9.8 m/s?
Use this kinematics equation:
2
2 0 2 9.8 m/s 2.5 m
7.0 m/s
i f
f
y yf i
y yf i
y f
y y y
a gv
v v a y
v v a y
v
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xx v t
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21
2f i x xi
x x v t a t
2 2
2x x xf iv v a x
x x xf iv v a t
Kinematics Equations
for
Constant Acceleration
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A train is approaching a town at a constant speed of 12 m/s. The
town is 1.0 km distant. After 30 seconds, the conductor applies the
breaks. What acceleration is necessary to bring the train to rest
exactly at the edge of town?
Example Problems
Slide 2-35
12 m/s, 30 siv t
31.0 km 1.0 10 m
?a 0fv
1d2d
1
2
2 2
2 2 2
2 2
12 m/s 30 s 360 m
1000 m 360 m 640 m
Now use 2 :
0 12 m/s0.1125 m/s 0.11 m/s
2 2 640 m
i
x xf i
x xf i
d v t
d
v v a x
v va
x
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The chapter begins and ends with a discussion of the possible outcome of a race between a
human and a horse. In fact, there have been some well publicized sprints between football
players and horses carrying jockeys. In order to render the contest more sporting, the human
is given a head start. We can make a rough model of a short sprint as a period of constant
acceleration followed by a period of constant speed. In this model, a very good human
sprinter can accelerate at 12 m/s² for 1.0 s; a horse can accelerate at 6.0 m/s² for 4.0 s.
Suppose a man and a horse of these abilities are competing in a race of 400 m; how much of
a head start would the man need in order to just tie the horse?
Slide 2-35
2
1
2
1
12 m/s , 1.0 s, ? , ?
6.0 m/s , 4.0 s, ? , ?
human human fhuman human
horse horse fhorse horse
a t v d
a t v d
400 m
? ?human horse
t t
1d
2 1400 md d
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Free Fall
Slide 2-36
Why is it negative?
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An arrow is launched vertically upward. It
moves straight up to a maximum height,
then falls to the ground. The trajectory of the
arrow is noted. Which choice below best
represents the arrow’s acceleration at the
different points?
Checking Understanding
A. A E B D; C 0
B. E D C B A
C. A B C D E
D. A B D E; C 0
Slide 2-37
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An arrow is launched vertically upward. It
moves straight up to a maximum height,
then falls to the ground. The trajectory of the
arrow is noted. Which choice below best
represents the arrow’s acceleration at the
different points?
Answer
A. A E B D; C 0
B. E D C B A
C. A B C D E
D. A B D E; C 0
Slide 2-38
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An arrow is launched vertically upward. It moves straight up to a
maximum height, then falls to the ground. The trajectory of the
arrow is noted. Which graph best represents the vertical velocity of
the arrow as a function of time? Ignore air resistance; the only force
acting is gravity.
Checking Understanding
Slide 2-39
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An arrow is launched vertically upward. It moves straight up to a
maximum height, then falls to the ground. The trajectory of the
arrow is noted. Which graph best represents the vertical velocity of
the arrow as a function of time? Ignore air resistance; the only force
acting is gravity.
Answer
D.
Slide 2-40
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The figure below shows five arrows with differing masses that were
launched straight up with the noted speeds. Rank the arrows, from
greatest to least, on the basis of the maximum height the arrows
reach. Ignore air resistance; the only force acting is gravity.
A. E D A B C
B. C D A B E
C. C B A D E
D. E B A D C
Slide 2-41
Checking Understanding
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The figure below shows five arrows with differing masses that were
launched straight up with the noted speeds. Rank the arrows, from
greatest to least, on the basis of the maximum height the arrows
reach. Ignore air resistance; the only force acting is gravity.
A. E D A B C
B. C D A B E
Slide 2-42
Answer
C. C B A D E
D. E B A D C
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Example Problems
Spud Webb, height 5'7", was one of the shortest basketball
players to play in the NBA. But he had an impressive vertical leap;
he was reputedly able to jump 110 cm off the ground. To jump this
high, with what speed would he leave the ground?
Slide 2-43
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Example Problems
A football is punted straight up into the air; it hits the ground
5.2 s later. What was the greatest height reached by the ball?
With what speed did it leave the kicker’s foot?
Slide 2-43
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Example Problems Passengers on The Giant
Drop, a free-fall ride at
Six Flags Great America,
sit in cars that are raised
to the top of a tower. The
cars are then released
for 2.6 s of free fall. How
fast are the passengers
moving at the end of this
speeding up phase of the
ride? If the cars in which
they ride then come to
rest in a time of 1.0 s,
what is acceleration
(magnitude and
direction) of this slowing
down phase of the ride?
Given these numbers,
what is the minimum
possible height of the
tower?
Slide 2-44
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Example Problems A pole vaulter is nearly motionless as he clears the bar, set 5.2 m above the ground. He then
falls onto a thick pad. The top of the pad is 75 cm above the ground; it compresses by 50 cm
as he comes to rest. What is his acceleration as he comes to rest on the pad?
Slide 2-44
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Summary
Slide 2-45
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Summary
Slide 2-46
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Additional Questions
A particle moves with the position-versus-time graph shown. Which
graph best illustrates the velocity of the particle as a function of
time?
A. B. C. D.
Slide 2-47
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A particle moves with the position-versus-time graph shown. Which
graph best illustrates the velocity of the particle as a function of
time?
Answer
A. B. C. D.
Slide 2-48
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A. P and Q have the same velocity at 2 s.
B. P and Q have the same velocity at 1 s and 3 s.
C. P and Q have the same velocity at 1 s, 2 s, and 3 s.
D. P and Q never have the same velocity.
Masses P and Q move with the
position graphs shown. Do P and
Q ever have the same velocity?
If so, at what time or times?
Additional Questions
Slide 2-49
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Masses P and Q move with the
position graphs shown. Do P and
Q ever have the same velocity?
If so, at what time or times?
Answer
A. P and Q have the same velocity at 2 s.
B. P and Q have the same velocity at 1 s and 3 s.
C. P and Q have the same velocity at 1 s, 2 s, and 3 s.
D. P and Q never have the same velocity.
Slide 2-50
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Additional Questions
Mike jumps out of a tree and lands on a trampoline. The
trampoline sags 2 feet before launching Mike back into the air.
At the very bottom, where the sag is the greatest, Mike’s
acceleration is:
A. Upward
B. Downward
C. Zero
Slide 2-51
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Answer
Mike jumps out of a tree and lands on a trampoline. The
trampoline sags 2 feet before launching Mike back into the air.
At the very bottom, where the sag is the greatest, Mike’s
acceleration is:
A. Upward
B. Downward
C. Zero
Slide 2-52
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Additional Example Problems
When you stop a car on a very slick icy pavement, the
acceleration of your car is approximately –1.0 m/s². If you are
driving on icy pavement at 30 m/s (about 65 mph) and hit your
brakes, how much distance will your car travel before coming to
rest?
Slide 2-53
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Additional Example Problems As we will see in a future
chapter, the time for a car to
come to rest in a collision is
always about 0.1 s. Ideally,
the front of the car will
crumple as this happens,
with the passenger
compartment staying intact.
If a car is moving at 15 m/s
and hits a fixed obstacle,
coming to rest in 0.10 s,
what is the acceleration?
How much does the front of
the car crumple during the
collision?
Slide 2-53