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Accelerated Motion

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Chapter Accelerated Motion 3 In this chapter you will: Develop descriptions of accelerated motions. Use graphs and equations to solve problems involving moving objects. – PowerPoint PPT presentation

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Title: Accelerated Motion


1
Accelerated Motion
Chapter
3
In this chapter you will
  • Develop descriptions of accelerated motions.
  • Use graphs and equations to solve problems
    involving moving objects.
  • Describe the motion of objects in free fall.

2
Table of Contents
Chapter
3
Chapter 3 Accelerated Motion
Section 3.1 Acceleration Section 3.2 Motion
with Constant Acceleration Section 3.3 Free Fall
  • Homework
  • Read Chapter 3. Complete Study Guide.
  • Graphical Analysis Packet
  • HW 3 handout

3
Warmup Which Velocity Is It?
Section
3.1
Physics Daily Warmup 16
  • There are two types of velocity that we encounter
    in our everyday lives. Instantaneous velocity
    refers to how fast something is moving at a
    particular point in time, while average velocity
    refers to the average speed something travels
    over a given period of time.
  • For each use of velocity described below,
    identify whether it is instantaneous velocity or
    average velocity.
  • The speedometer on your car indicates you are
  • going 65 mph.
  • A race-car driver was listed as driving 120 mph
  • for the entire race.
  • A freely falling object has a speed of 19.6 m/s
    after
  • 2 seconds of fall in a vacuum.
  • 4. The speed limit sign says 45 mph.

instantaneous
average
instantaneous
instantaneous
4
Acceleration
Section
3.1
In this section you will
  • Define acceleration.
  • Relate velocity and acceleration to the motion of
    an object.
  • Create velocity-time graphs.

5
Acceleration
Section
3.1
Changing Velocity
  • You can feel a difference between uniform and
    nonuniform motion.
  • When you move in
  • nonuniform motion, you feel pushed or pulled.
  • In contrast, when you are in
  • uniform motion and your eyes are closed, you
    feel as though you are not moving at all.

6
Acceleration
Section
3.1
Changing Velocity
  • Consider the motion diagrams below showing the
    distance between successive positions.

7
Acceleration
Section
3.1
Changing Velocity
  • There are two major indicators of the change in
    velocity in this motion diagram. The change in
    the spacing of the stick figures or dots and the
    differences in the lengths of the velocity
    vectors indicate the changes in velocity.

8
Acceleration
Section
3.1
Changing Velocity
  • If an object speeds up, each subsequent velocity
    vector is longer.
  • If the object slows down, each vector is shorter
    than the previous one.
  • Both types of motion diagrams give an idea of how
    an objects velocity is changing.

9
Acceleration
Section
3.1
Velocity-Time Graphs
Play ch3_1_movanim.
10
Acceleration
Section
3.1
Average and Instantaneous Acceleration
  • The rate at which an objects velocity changes is
    called the acceleration of the object. When the
    velocity of an object changes at a constant rate,
    it has a constant acceleration.
  • The average acceleration of an object is the
    change in velocity during some measurable time
    interval divided by that time interval.
  • Average acceleration is measured in m/s2.
  • The change in velocity at an instant of time is
    called instantaneous acceleration.

11
Acceleration
Section
3.1
Instantaneous Acceleration
  • The instantaneous acceleration of an object can
    be found by
  • drawing a tangent line on the velocity-time
    graph at the point of time in which you are
    interested.
  • The slope of this line is equal to the
    instantaneous acceleration.

Example Tangent line is drawn at t 1.0 s and t
5.0 s
12
Acceleration
Section
3.1
Average Acceleration
  • The average acceleration of an object can be
    found by plotting two points which define the
    time interval, connecting the points with a
    straight line, ad finding the slope of the line.

Example slope of the line indicates the average
velocity between 1.0 and 5.0 s.
13
Section
Velocity Time Graph Example
3.1
a) How would you describe the sprinters velocity
and acceleration as shown on the graph? From the
graph, note that the sprinters velocity starts
at zero, increases rapidly for the first few
seconds, and then, after reaching about 10.0 m/s,
remains almost constant.
14
Section
Velocity Time Graph Example
3.1
b) What is his instantaneous acceleration at t
1 second and t 5 seconds? Instantaneous
acceleration Draw a tangent to the curve at t
1.0 s and t 6.0 s. Find the slope of each line.
15
Section
Velocity Time Graph Example
3.1
Solve for acceleration at 1.0 s
Solve for acceleration at 5.0 s
The acceleration is not constant because it
changes from 3.4 m/s2 to 0.03 m/s2 at 5.0 s. The
acceleration is in the direction chosen to be
positive because both values are positive.
16
Section
Velocity Time Graph Example
3.1
c) What is his average acceleration between 1 and
5 seconds? Average acceleration Draw a line
between the points at t 1 s and t 5 s. Find
the slope of the line.
The slope is 4 m/s 5 s 0.8 m/s2
17
Acceleration
Section
3.1
Positive and Negative Acceleration
  • These four motion diagrams represent the four
    different possible ways to move along a straight
    line with constant acceleration.
  • Positive direction, speeding up
  • Positive direction, slowing down
  • Negative direction, speeding up
  • Negative direction, slowing down

18
Acceleration
Section
3.1
Positive and Negative Acceleration
  • When the object is speeding up, the velocity and
    acceleration vectors point in the same direction.
    (case 1 and 3)
  • When the object is slowing down, the velocity and
    acceleration vectors point in opposite directions
    (case 2 and 4)
  • Both the direction of an objects velocity and
    its direction of acceleration are needed to
    determine whether it is speeding up or slowing
    down.

19
Acceleration
Section
3.1
Positive and Negative Acceleration
  • An object has a positive acceleration when the
    acceleration vector points in the positive
    direction and a negative acceleration, when the
    acceleration vector points in the negative
    direction.
  • The sign of acceleration does not indicate
    whether the object is speeding up or slowing down.

20
Acceleration
Section
3.1
Determining Acceleration from a v-t Graph
  • Velocity and acceleration information also is
    contained in velocity-time graphs.
  • Graphs A, B, C, D, and E, as shown on the right,
    represent the motions of five different runners.
  • Positive velocity in this graph means the
    direction is east. Negative velocity means the
    direction is west.

21
Section
Determining Acceleration from a v-t Graph
3.1
  • Describe the direction of motion, velocity and
    acceleration for
  • Graph A
  • Zero slope means zero acceleration constant
    velocity towards the east.
  • Graph B
  • Starting with zero velocity, picking up speed,
    moving towards the east. The straight line
    indicates constant acceleration.
  • Graph C
  • Moving towards the east while slowing down and
    eventually stopping. Slowing down with a constant
    negative acceleration.

22
Section
Determining Acceleration from a v-t Graph
3.1
  • Graph D
  • Moving towards the west while slowing down, turns
    around, then moves east while speeding up.
  • Graph E
  • Moving west with constant velocity, zero
    acceleration.
  • Remember, for a velocity-time graph
  • - positive velocity occurs when the line is
    anywhere above the x-axis
  • - positive velocity means the object is moving
    in the positive direction, which might be
    east, to the right, etc.
  • - the slope of the line indicated the
    acceleration
  • - a straight line means constant acceleration

23
Acceleration
Section
3.1
Determining Acceleration from a v-t Graph
  • The following equation expresses average
    acceleration as the slope of the velocity-time
    graph.
  • Average acceleration is equal to the change in
    velocity, divided by the time it takes to make
    that change.

24
Acceleration
Section
3.1
Example Suppose you run a wind sprints back
and forth across the gym. You run at a speed of
4.0 m/s toward the wall, touch, and run back at
the same speed. The whole trip takes 10 seconds.
What is your average acceleration if the positive
direction is toward the wall?
Givens vi 4 m/s vf -4 m/s ?t 10
s Unknown a Equation Substitute and
Solve Sense The negative sign means the
acceleration is away from the wall. Change in
direction of motion results in acceleration.
25
Section Check
Section
3.1
Question 1
  • Which of the following statements correctly
    define acceleration?
  1. Acceleration is the rate of change of
    displacement of an object.
  2. Acceleration is the rate of change of velocity of
    an object.
  3. Acceleration is the amount of distance covered in
    unit time.
  4. Acceleration is the rate of change of speed of an
    object.

26
Section Check
Section
3.1
Answer 1
  • Answer B

Reason The rate at which an objects velocity
changes is called acceleration of the object.
27
Section Check
Section
3.1
Question 2
  • What happens when the velocity vector and the
    acceleration vector of an object in motion are in
    same direction?
  1. The acceleration of the object increases.
  2. The speed of the object increases.
  3. The object comes to rest.
  4. The speed of the object decreases.

28
Section Check
Section
3.1
Answer 2
  • Answer B

Reason When the velocity vector and the
acceleration vector of an object in motion are in
same direction, the speed of the object increases.
29
Section Check
Section
3.1
Question 3
  • On the basis of the velocity-time graph of a car
    moving up a hill, as shown on the right,
    determine the average acceleration of the car?
  1. 0.5 m/s2
  2. -0.5 m/s2
  1. 2 m/s2
  2. -2 m/s2

30
Section Check
Section
3.1
Answer 3
  • Answer B

Reason Average acceleration of an object is the
slope of the velocity-time graph.
vf 0 vi 25 m/s tf 50 s ti 0 a vf
vi 0 25 m/s - 0.5 m/s2 tf ti 50
s - 0
31
Section Check
Section
3.2
Practice Problems, p. 64 6, 7, 9, 10.
Motion with Constant Acceleration
Steel Ball Race, p. 58
32
Motion with Constant Acceleration
Section
3.2
In this section you will
  • Interpret position-time graphs for motion with
    constant acceleration.
  • Determine mathematical relationships among
    position, velocity, acceleration, and time.
  • Apply graphical and mathematical relationships to
    solve problems related to constant acceleration.

33
Motion with Constant Acceleration
Section
3.2
Velocity with Average Acceleration
  • If an objects average acceleration during a time
    interval is known, then it can be used to
    determine how much the velocity changed during
    that time.
  • The definition of average acceleration

can be rewritten as follows
34
Motion with Constant Acceleration
Section
3.2
Velocity with Average Acceleration
  • The equation for final velocity with average
    acceleration can be written as follows
  • The final velocity is equal to the initial
    velocity plus the product of the average
    acceleration and time interval.

vi
35
Motion with Constant Acceleration
Section
3.2
Velocity with Average Acceleration
  • In cases in which the acceleration is constant,
    the average acceleration, a, is the same as the
    instantaneous acceleration, a.
  • The equation for final velocity can be rearranged
    to find the time at which an object with constant
    acceleration has a given velocity.
  • It also can be used to calculate the initial
    velocity of an object when both the velocity and
    the time at which it occurred are given.

36
Motion with Constant Acceleration
Section
3.2
Position with Constant Acceleration
  • The position data at different time intervals for
    a car with constant acceleration are shown in the
    table.
  • The data from the table are graphed as shown on
    the next slide.

37
Motion with Constant Acceleration
Section
3.2
Position with Constant Acceleration
  • The graph shows that the cars motion is not
    uniform the displacements for equal time
    intervals on the graph get larger and larger.
  • The slope of a position-time graph of a car
    moving with a constant acceleration gets steeper
    as time goes on.

38
Motion with Constant Acceleration
Section
3.2
Position with Constant Acceleration
  • The slopes from the position time graph can be
    used to create a velocity-time graph as shown on
    the right.
  • Note that the slopes shown in the position-time
    graph are the same as the velocities graphed in
    velocity-time graph.

39
Motion with Constant Acceleration
Section
3.2
Position with Constant Acceleration
  • A velocity-time graph does not contain any
    information about the objects position.
  • However, the velocity-time graph does contain
    information about the objects displacement.
  • Recall that for an object moving at a constant
    velocity,

40
Motion with Constant Acceleration
Section
3.2
Position with Constant Acceleration
  • On the graph shown on the right, v is the height
    of the plotted line above the t-axis, while ?t is
    the width of the shaded rectangle. The area of
    the rectangle, then, is v?t, or ?d. Thus, the
    area under the v-t graph is equal to the objects
    displacement.
  • The area under the v-t graph is equal to the
    objects displacement.

41
Motion with Constant Acceleration
Section
3.2
Finding the Displacement from a v-t Graph Example
1
The v-t graph below shows the motion of an
airplane. Find the displacement of the airplane
at ?t 1.0 s and at ?t 2.0 s.
42
Motion with Constant Acceleration
Section
3.2
Finding the Displacement from a v-t Graph
The displacement is the area under the v-t graph.
The time intervals begin at t 0.0.
43
Motion with Constant Acceleration
Section
3.2
Finding the Displacement from a v-t Graph
Identify the given and unknown variables.
Given v 75 m/s ?t 1.0 s ?t 2.0 s
Unknown ?d ?
44
Motion with Constant Acceleration
Section
3.2
Finding the Displacement from a v-t Graph
Solve for displacement during ?t 1.0
s. Equation
Substitute v 75 m/s, ?t 1.0 s Solve
45
Motion with Constant Acceleration
Section
3.2
Finding the Displacement from a v-t Graph
Solve for displacement during ?t 2.0 s.
Substitute v 75 m/s, ?t 2.0 s
46
Motion with Constant Acceleration
Section
3.2
Finding the Displacement from a v-t Graph Sense
  • Are the units correct?
  • Displacement is measured in meters.
  • Do the signs make sense?
  • The positive sign agrees with the graph.
  • Is the magnitude realistic?
  • Moving a distance to about one football field is
    reasonable for an airplane.

47
Motion with Constant Acceleration
Section
3.2
An Alternative Expression
  • Often, it is useful to relate position, velocity,
    and constant acceleration without including time.
  • The three equations for motion with constant
    acceleration are summarized in the table.

48
Section Check
Section
3.2
Question 1
  • A position-time graph of a bike moving with
    constant acceleration is shown on the right.
    Which statement is correct regarding the
    displacement of the bike?
  1. The displacement in equal time interval is
    constant.
  2. The displacement in equal time interval
    progressively increases.
  1. The displacement in equal time interval
    progressively decreases.
  2. The displacement in equal time interval first
    increases, then after reaching a particular point
    it decreases.

49
Section Check
Section
3.2
Answer 1
  • Answer B

Reason You will see that the slope gets steeper
as time goes, which means that the displacement
in equal time interval progressively gets larger
and larger.
50
Section Check
Section
3.2
Question 2
  • A car is moving with an initial velocity of vi
    m/s. After reaching a highway, it moves with a
    constant acceleration of a m/s2, what will be the
    velocity (vf) of the car after traveling for t
    seconds?
  1. vf vi at
  2. vf vi 2at
  3. vf2 vi2 2at
  4. vf vi at

51
Section Check
Section
3.2
Answer 2
  • Answer A

Reason Since a ?v/?t vf - vi a (tf -
ti) Also since car is starting from rest, ti
0 Therefore vf vi at (where t is the total
time)
52
Section
Section Check
3.2
Question 3
If you were given initial and final velocities
and the constant acceleration of an object, and
you were asked to find the displacement, what
equation would you use?
  1. vf vi at
  2. df di vi t ½ at2
  3. vf2 vi2 2a(df - di)
  4. vf vi at

53
Section
Section Check
3.2
Answer 3
Answer C
Reason Kinematics equation number 3 does not
require time. You are not given time in the
problem.
54
Motion with Constant Acceleration
Section
3.2
Practice Problems p.65 20, 21. Section Review
p.71 34, 39.
55
Free Fall
Section
3.3
Free Fall
In this section you will
  • Define acceleration due to gravity.
  • Solve problems involving objects in free fall.

56
Free Fall
Section
3.3
Acceleration Due to Gravity
  • free fall the motion of a body when air
    resistance is negligible and the motion can be
    considered due to the force of gravity alone.
  • After a lot of observation, Galileo concluded
    that, neglecting the effect of the air, all
    objects in free fall had the same acceleration.
  • It didnt matter what they were made of, how
    much they weighed, what height they were dropped
    from, or whether they were dropped or thrown.
  • The acceleration of falling objects, given a
    special symbol, g, is equal to 9.80 m/s2.
  • The acceleration due to gravity is the
    acceleration of an object in free fall that
    results from the influence of Earths gravity.

57
Free Fall
Section
3.3
Acceleration Due to Gravity
ch 3_4_movanim
58
Free Fall
Section
3.3
Acceleration Due to Gravity
  • At the top of the flight, the balls velocity is
    0 m/s. What would happen if its acceleration were
    also zero? Then, the balls velocity would not be
    changing and would remain at 0 m/s.
  • If this were the case, the ball would not gain
    any downward velocity and would simply hover in
    the air at the top of its flight.
  • Because this is not the way objects tossed in the
    air behave on Earth, you know that the
    acceleration of an object at the top of its
    flight must not be zero. Further, because you
    know that the object will fall from that height,
    you know that the acceleration must be downward.

59
Free Fall
Section
3.3
Acceleration Due to Gravity
  • Amusement parks use the concept of free fall to
    design rides that give the riders the sensation
    of free fall.
  • These types of rides usually consist of three
    parts the ride to the top, momentary suspension,
    and the plunge downward.
  • When the cars are in free fall, the most massive
    rider and the least massive rider will have the
    same acceleration.

60
Free Fall
Section
3.3
Acceleration Due to Gravity
  • Example Suppose the free-fall ride at an
    amusement park starts at rest and is in free fall
    for 1.5 s. What would be its velocity at the end
    of 1.5 s? How far would it fall?
  • Choose a coordinate system with a positive axis
    upward and the origin at the initial position of
    the car. Because the car starts at rest, vi would
    be equal to 0.0 m/s.
  • Givens vi 0.0 m/s, di 0.0 m, ti 0.0 s, tf
    1.5 s, a -9.8 m/s2
  • Unknown vf

61
Free Fall
Section
3.3
Acceleration Due to Gravity
  • To calculate the final velocity, use the equation
    for velocity with constant acceleration.
  • Equation
  • Substitute Solve
  • Sense Negative velocity means down units are
    OK.

62
Free Fall
Section
3.3
Acceleration Due to Gravity
  • How far does the car fall? Use the equation for
    displacement when time and constant acceleration
    are known.
  • Equation
  • Substitute Solve
  • Sense The displacement is negative because it
    fell, and the units are meters. Looks good!

63
Section Check
Section
3.3
Question 1
  • What is free fall?

64
Section Check
Section
3.3
Answer 1
  • Free Fall is the motion of the body when air
    resistance is negligible and the action can be
    considered due to gravity alone.

65
Section Check
Section
3.3
Question 2
  • If a stone is thrown vertically upwards with a
    velocity of 25 m/s, what will be the velocity of
    the stone after 1 second?
  1. 9.8 m/s
  2. 15.2 m/s
  3. 25 m/s
  4. 34.8 m/s

66
Section Check
Section
3.3
Answer 2
  • Answer B

Reason Since the ball is thrown upwards, the
velocity and acceleration are in opposite
directions, therefore the speed of the ball
decreases. After 1 s, the balls velocity is
reduced by 9.8 m/s (as acceleration due to
gravity is 9.8 m/s2), so it is now traveling at
25 m/s 9.8 m/s 15.2 m/s.
67
Section Check
Section
3.3
Question 3
  • If a 50-kg bag and a 100-kg bag are dropped from
    a height of 50 m. Which of the following
    statement is true about their acceleration?
    (Neglect air resistance)
  1. 100-kg bag will fall with a greater acceleration.
  2. 50-kg bag will fall with a greater acceleration.
  3. Both will fall at the same and constant rate of
    acceleration.
  4. Both will fall at the same rate of acceleration,
    which changes equally as time goes.

68
Section Check
Section
3.3
Answer 3
  • Answer C

Reason Any body falling freely towards Earth,
falls with a same and constant acceleration of
9.8 m/s2. It doesnt matter how much it weighed
and what height it was dropped from.
69
Free Fall
Section
3.3
  • Exit Ticket Please write in full sentences.
  • Describe the velocity and acceleration of a ball
    that is tossed in the air and comes back down.

70
Free Fall
Section
3.3
  • Practice Problems p.74 42, 44.
  • HW 3 handout.

71
Chapter
Physics Chapter 2 3 Test Information The test
is worth 46 points total. Matching 12
questions, 12 points total Problems 7 questions,
34 points total Know - vocabulary for both
chapters - how to interpret and draw
position-time and velocity-time graphs - how
to use the 3 kinematics equations to solve
problems - how to express answers with correct
units and sig figs
3
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