Kinematics in One Dimension

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Kinematics in One Dimension
Chapter 2
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Kinematics deals with the concepts that are
needed to describe motion. Dynamics deals with
the effect that forces have on motion. Together,
kinematics and dynamics form the branch of
physics known as Mechanics.
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2.1 Displacement
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2.1 Displacement
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2.1 Displacement
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2.1 Displacement
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2.2 Speed and Velocity
Average speed is the distance traveled divided by
the time required to cover the distance.
SI units for speed meters per second (m/s)
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2.2 Speed and Velocity
Example 1 Distance Run by a Jogger How far does
a jogger run in 1.5 hours (5400 s) if his
average speed is 2.22 m/s?
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2.2 Speed and Velocity
Average velocity is the displacement divided by
the elapsed time.
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2.2 Speed and Velocity
Example 2 The Worlds Fastest Jet-Engine
Car Andy Green in the car ThrustSSC set a world
record of 341.1 m/s in 1997. To establish such
a record, the driver makes two runs through the
course, one in each direction, to nullify wind
effects. From the data, determine the
average velocity for each run.
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2.2 Speed and Velocity
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2.3 Acceleration
The notion of acceleration emerges when a change
in velocity is combined with the time during
which the change occurs.
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2.3 Acceleration
DEFINITION OF AVERAGE ACCELERATION
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2.3 Acceleration
Example 3 Acceleration and Increasing
Velocity Determine the average acceleration of
the plane.
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2.3 Acceleration
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2.3 Acceleration
Example 3 Acceleration and Decreasing Velocity
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2.4 Equations of Kinematics for Constant
Acceleration
Equations of Kinematics for Constant Acceleration
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2.4 Equations of Kinematics for Constant
Acceleration
Five kinematic variables 1. displacement, x 2.
acceleration (constant), a 3. final velocity (at
time t), v 4. initial velocity, vo 5. elapsed
time, t
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2.4 Equations of Kinematics for Constant
Acceleration
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2.4 Equations of Kinematics for Constant
Acceleration
Example 6 Catapulting a Jet Find its
displacement.
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2.4 Equations of Kinematics for Constant
Acceleration
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2.5 Applications of the Equations of Kinematics
Reasoning Strategy 1. Make a drawing. 2. Decide
which directions are to be called positive ()
and negative (-). 3. Write down the values
that are given for any of the five kinematic
variables. 4. Verify that the information
contains values for at least three of the five
kinematic variables. Select the appropriate
equation. 5. When the motion is divided into
segments, remember that the final velocity of one
segment is the initial velocity for the next. 6.
Keep in mind that there may be two possible
answers to a kinematics problem.
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2.5 Applications of the Equations of Kinematics
Example 8 An Accelerating Spacecraft A
spacecraft is traveling with a velocity of 3250
m/s. Suddenly the retrorockets are fired, and
the spacecraft begins to slow down with an
acceleration whose magnitude is 10.0 m/s2. What
is the velocity of the spacecraft when the
displacement of the craft is 215 km, relative to
the point where the retrorockets began firing?
x a v vo t
215000 m -10.0 m/s2 ? 3250 m/s
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2.5 Applications of the Equations of Kinematics
x a v vo t
215000 m -10.0 m/s2 ? 3250 m/s
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2.6 Freely Falling Bodies
In the absence of air resistance, it is found
that all bodies at the same location above the
Earth fall vertically with the same
acceleration.
This idealized motion is called free-fall and the
acceleration of a freely falling body is called
the acceleration due to gravity.
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2.6 Freely Falling Bodies
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2.6 Freely Falling Bodies
Example 10 A Falling Stone A stone is dropped
from the top of a tall building. After 3.00s of
free fall, what is the displacement y of the
stone?
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2.6 Freely Falling Bodies
y a v vo t
? -9.80 m/s2 0 m/s 3.00 s
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2.6 Freely Falling Bodies
y a v vo t
? -9.80 m/s2 0 m/s 3.00 s
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2.6 Freely Falling Bodies
Example 12 How High Does it Go? The referee
tosses the coin up with an initial speed of
5.00m/s. In the absence if air resistance, how
high does the coin go above its point of release?
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2.6 Freely Falling Bodies
y a v vo t
? -9.80 m/s2 0 m/s 5.00 m/s
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2.6 Freely Falling Bodies
y a v vo t
? -9.80 m/s2 0 m/s 5.00 m/s
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2.6 Freely Falling Bodies
Conceptual Example 14 Acceleration Versus
Velocity There are three parts to the motion of
the coin. On the way up, the coin has a vector
velocity that is directed upward and has
decreasing magnitude. At the top of its path, the
coin momentarily has zero velocity. On the way
down, the coin has downward-pointing velocity
with an increasing magnitude. In the absence of
air resistance, does the acceleration of
the coin, like the velocity, change from one part
to another?
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2.6 Freely Falling Bodies
Conceptual Example 15 Taking Advantage of
Symmetry Does the pellet in part b strike the
ground beneath the cliff with a smaller, greater,
or the same speed as the pellet in part a?
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Position-Time Graphs

• We can use a postion-time graph to illustrate the
  motion of an object.
• Postion is on the y-axis
• Time is on the x-axis

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Plotting a Distance-Time Graph

• Axis
• Distance (position) on y-axis (vertical)
• Time on x-axis (horizontal)
• Slope is the velocity
• Steeper slope faster
• No slope (horizontal line) staying still

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Where and When

• We can use a position time graph to tell us where
  an object is at any moment in time.
• Where was the car at 4 s?
• 30 m
• How long did it take the car to travel 20 m?
• 3.2 s

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Interpret this graph
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Describing in Words
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Describing in Words

• Describe the motion of the object.
• When is the object moving in the positive
  direction?
• Negative direction.
• When is the object stopped?
• When is the object moving the fastest?
• The slowest?

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Accelerated Motion

• In a position/displacement time graph a straight
  line denotes constant velocity.
• In a position/displacement time graph a curved
  line denotes changing velocity (acceleration).
• The instantaneous velocity is a line tangent to
  the curve.

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Accelerated Motion

• In a velocity time graph a line with no slope
  means constant velocity and no acceleration.
• In a velocity time graph a sloping line means a
  changing velocity and the object is accelerating.

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Velocity

• Velocity changes when an object
• Speeds Up
• Slows Down
• Change direction

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Velocity-Time Graphs

• Velocity is placed on the vertical or y-axis.
• Time is place on the horizontal or x-axis.
• We can interpret the motion of an object using a
  velocity-time graph.

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Constant Velocity

• Objects with a constant velocity have no
  acceleration
• This is graphed as a flat line on a velocity time
  graph.

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Changing Velocity

• Objects with a changing velocity are undergoing
  acceleration.
• Acceleration is represented on a velocity time
  graph as a sloped line.

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Positive and Negative Velocity

• The first set of graphs show an object traveling
  in a positive direction.
• The second set of graphs show an object traveling
  in a negative direction.

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Speeding Up and Slowing Down

• The graphs on the left represent an object
  speeding up.
• The graphs on the right represent an object that
  is slowing down.

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Two Stage Rocket

• Between which time does the rocket have the
  greatest acceleration?
• At which point does the velocity of the rocket
  change.

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Displacement from a Velocity-Time Graph

• The shaded region under a velocity time graph
  represents the displacement of the object.
• The method used to find the area under a line on
  a velocity-time graph depends on whether the
  section bounded by the line and the axes is a
  rectangle, a triangle

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2.7 Graphical Analysis of Velocity and
Acceleration
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2.7 Graphical Analysis of Velocity and
Acceleration
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2.7 Graphical Analysis of Velocity and
Acceleration