Describing Constant Motion

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Section 8.1

• Describing Constant Motion

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Frame of reference

• We can tell something is moving because it is
  moving past something else.
• Ex we know a runner is moving because he passes
  the bleachers.
• The bleachers would be considered a point of
  reference - background used to determine motion

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Frame of reference

• Our most common frame of reference is the earth
  or something attached to it (tree etc..)
• Earth not always the best reference.
• Ex Sun appears to move up over the earth. Does
  the sun move past the earth?

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Motion

• Motion is any change in location or position of
  an object.
• The distance an object as moved from one point to
  another is the distance traveled.
• Distance traveled is usually measured in meters
  or kilometers (metric system)

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Example

• Doug ran 15 meters from the start line during the
  race.
• What is the distance traveled?
• 15 meters (always include the units numbers are
  meaningless without units)

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Another important fact.

• When running a race, it is important to know how
  fast the runner ran.
• Usually measured in minutes or seconds

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Example

• Doug ran the race in 3 seconds.
• How quickly did Doug run the race?
• 3 seconds (here again use units with your
  numbers)

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Rate

• We measure distance and time to determine the
  rate at which something is traveling.
• Ex speed of a car measure with a speedometer.
• We will use the Metric system meters/second or
  m/s.

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Speed

• The distance that an object moves per unit of
  time.
• Speed distance traveled
• time

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Speed - Dougs example

• What we know
• the distance Doug traveled was 15 meters.
• Doug ran the race in 3 seconds.
• Plug this into the formula
• Speed distance traveled d
• time t
• Speed 15 meters 15m 5m/s
• 3 seconds 3s

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Velocity

• This is basically the same as speed except that
  you describe the motion with a direction.
• Velocity distance traveled in a given direction
• time
• Doug ran 5 m/s North (5 m/s N)

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Questions Copy these down and answer

• What is needed to decide if an object is moving?
• Give two examples of moving objects and their
  backgrounds used as frame of reference.
• What two characteristics of motion are needed to
  find speed?
• What additional information does velocity
  include?
• What is the speed of a car that travels 180
  meters in 6 seconds? (Show the calculations) If
  it does this while traveling south, what is its
  velocity?