Chapter 5 Matter in Motion

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Chapter 5Matter in Motion

• Section 1
• Measuring Matter

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Measuring Motion

• Motion is one of the key topics in physics.
• Everything in the universe moves. It might
• only be a small amount and very slow, but
• movement does happen. Don't forget that
• even if you are standing still, the Earth is
• moving around the Sun and the Sun is
• moving around our galaxy. The movement
• never stops.

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The Domino Derby

• Open Your Book to page 107 and in your DOS try to
  respond to the 4 questions listed in What Do You
  Think?
• Well revisit them later.
• Now Lets Investigate!

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Measuring Motion

• How do we know something is in motion?

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Observing motion

• Things are constantly in motion all around
  useven if we cannot see them.
• We can observe an object in motion when we
  compare it to a reference point that stays still.
  We can then describe the motion in terms of
  direction such as north, south, east, or west.

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Observing motion

• Sometimes a reference point is moving for
  example, when you see an airplane flying through
  the sky while you are riding in a car. You can
  tell the airplane is in motion because it changes
  position in relation to you.

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What is Speed?

• Speed is a scalar quantity (Scalars are
  quantities which are fully described by a
  magnitude (or numerical value) alone. ) which
  refers to "how fast an object is moving." Speed
  can be thought of as the rate at which an object
  covers distance.
• A fast-moving object has a high speed and covers
  a relatively large distance in a short amount of
  time.
• A slow-moving object has a low speed and covers a
  relatively small amount of distance in a short
  amount of time.
• An object with no movement at all has a zero
  speed.

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Speed depends on Distance and Time

• The rate at which an object moves is its speed.
• Speed depends on the distance traveled and the
  time taken to travel that distance.

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Average v Instantaneous Speed

• Animation

Instantaneous Speed - the speed at any given
instant in time. Average Speed - the average of
all instantaneous speeds found simply by a
distance/time ratio.
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Lets practice a little more

• If you walk for 1.5 hours and travel 7.5 km, what
  is your average speed?
• A bird flies at a speed of 15m/s for 10 s, 20 m/s
  for 10 s, and 25 m/s for 5 s. What is the birds
  average speed?

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Recognizing Speed on a Graph

• This graph shows distance traveled over time.
  Notice time is the independent variable and is
  graphed on the x-axis
• p109

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Velocity

• Velocity is a vector quantity (Vectors are
  quantities which are fully described by both a
  magnitude and a direction. )which refers to "the
  rate at which an object changes its position."

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Velocity Direction Matters

• The speed of an object in a particular direction
  is the objects velocity.
• SPEED AND VELOCITY DO NOT MEAN THE SAME THING!
  Velocity must include a reference direction. For
  example, the car is traveling 650 km/h south.

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Determining Resultant Velocity
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Acceleration

• Acceleration is a vector quantity which is
  defined as the rate at which an object changes
  its velocity. An object is accelerating if it is
  changing its velocity.
• Sports announcers will occasionally say that a
  person is accelerating if he/she is moving fast
  or speeding up. Yet acceleration has nothing to
  do with going fast. A person can be moving very
  fast and still not be accelerating.

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Acceleration

• Acceleration has to do with changing how fast an
  object is moving. If an object is not changing
  its velocity, then the object is not
  accelerating.
• Remember that velocity changes if the speed
  changes, or the direction changes, or if both
  change

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Acceleration

• The data at the right are representative of a
  northward-moving accelerating object. The
  velocity is changing over the course of time. In
  fact, the velocity is changing by a constant
  amount - 10 m/s - in each second of time. Anytime
  an object's velocity is changing, the object is
  said to be accelerating it has an acceleration.
  (animation)

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Examples of Acceleration
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Acceleration or not, and why?

• 1. You are riding your bike at 9 km/h. Ten
  minutes later your speed is 6km/h.
• You ride your bike around the block at a constant
  speed of 11 km/h.
• You ride your bike in a straight line at a
  constant speed of 10 km/h.

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Average Acceleration

• The average acceleration (a) of any object over a
  given interval of time (t) can be calculated
  using the equation

f final iinitial or final vvelocity
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Acceleration Values
Acceleration values are expressed in units of
velocity/time. Typical acceleration units include
the following m/s/s mi/hr/s km/hr/s m/s2
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Visual Representations

• Like the study of all of physics, our study of
  1-dimensional kinematics will be concerned with
  the multiple means by which the motion of objects
  can be represented. Such means include the use of
  words, the use of graphs, the use of numbers, the
  use of equations, and the use of diagrams.

Kinematics is the science of describing the
motion of objects using words, diagrams, numbers,
graphs, and equations.
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Position v Time Graphs

• To begin, consider a car moving with a constant,
  rightward () velocity - say of 10 m/s.

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Position v Time Graphs

• If the position-time data for such a car were
  graphed, then the resulting graph would look like
  the graph at the right. Note that a motion
  described as a constant, positive velocity
  results in a line of constant and positive slope
  when plotted as a position-time graph.

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Position v Time Graphs

• Now consider a car moving with a rightward (),
  changing velocity - that is, a car that is moving
  rightward but speeding up or accelerating.

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Position v Time Graphs

• If the position-time data for such a car were
  graphed, then the resulting graph would look like
  the graph at the right. Note that a motion
  described as a changing, positive velocity
  results in a line of changing and positive slope
  when plotted as a position-time graph.

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Position v Time Graphs
Constant Velocity Positive Velocity Positive Velocity Changing Velocity (acceleration)