Chapter 3 Describing Motion

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Chapter 3 Describing Motion

• In this chapter, you will begin to use tools to
  analyze motion in terms of displacement,
  velocity, and acceleration.
• These concepts will help you to determine how
  fast and how far an object will move, whether the
  object is speeding up or slowing down, and
  whether it is standing still or moving at a
  constant speed.

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Picture Motion

• When an object is in motion, its position
  changes, and that its position can changed along
  the path of a straight line, a circle, a graceful
  arc, or a back and forth vibration.
• Motion Diagrams A series of images of a moving
  object that records its position after equal time
  intervals.
• Examples are Fig. 3-5 pg. 45.
• 4 parts of motion
• At rest
• Speeding up
• Slowing Down
• Constant speed

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Particle Motion

• Replacing an object by a single point is called
  the particle model.
• Look at Figure 3-6. page 46.
• Use the particle model to draw a motion diagram
  for a car that starts from rest, speeds up to a
  constant speed, and then slows to a stop.

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• Scalar Quantity A quantity that only has
  magnitude.
• Vector Quantity A quantity that has both
  magnitude and direction.
• Scalar quantities are represented by simple
  letters such as m, t, and T.
• Vector quantities tells you not only the
  magnitude of the quantity, but also the direction.

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• Displacement defines the distance and direction
  between two positions.
• The difference between two times is called the
  time interval.

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• If it takes a sprinter to run the 100 meter-dash
  in 9.68 seconds, what was his time interval?
• Distance a scalar quantity that is the length,
  or size, of the displacement vector.

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• Remember Distance is a scalar quantity.
• Draw a dot diagram of a car speeding up.
• Add displacement vectors to the diagram.
• Draw a dot diagram for a runner slowing down to a
  stop.
• Add displacement vectors between each dots.
• Draw a diagram of a bus that speeds up, then
  moves at constant speed, then brakes to a halt.
• Add displacement vectors.

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• Velocity Speed and direction. (sound familiar?)
• Suppose you record a speedy jogger and a slow
  jogger, you will be able to tell that the
  position of the jogger changes more than that of
  the walker. In other words, for a fixed time
  interval, the displacement is larger for the
  jogger because she is moving faster.

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Average Velocity

• To get average velocity, you need both distance
  and time.

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Average Speed

• Average speed is the ratio of the total distance
  traveled to the time interval.
• Example Automobile speeds are measured in miles
  per hour, but in physics, things will be measured
  in meters per second. (m/s)

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Instantaneous Velocity

• Instantaneous Velocity The speed and direction
  of an object at a particular unit of time.

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Displacement Average Velocity and Time
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Acceleration

• An object in motion whose velocity is changing is
  said to be accelerating.
• Recall that an objects velocity changes when
  either the magnitude or direction of motion
  changes.
• Average Acceleration The change in average
  velocity divided by the time.

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Average Acceleration