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Physics 121: Fundamentals of Physics I. September 13, 2006. Interpreting Position vs. Time Graphs. Steeper slopes correspond to faster speed Flat implies standing still Negative slopes -> negative velocity (motion to the left)

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physics 121 fundamentals of physics i

Physics 121:Fundamentals of Physics I

September 13, 2006

University of Maryland

interpreting position vs time graphs
Interpreting Position vs. Time Graphs
  • Steeper slopes correspond to faster speed
    • Flat implies standing still
  • Negative slopes -> negative velocity (motion to the left)
  • The slope is the ratio of intervals, Dx/Dt, not a ratio of coordinates (notx/t)
  • Be sure to look at the scale on the x-axis (time) and y-axis (position)
    • Also, make sure you know where the origin is
    • Positive position means to right of origin
    • Negative position means to left of origin

University of Maryland

interpreting velocity vs time graphs
Interpreting Velocity vs. Time Graphs
  • Positive velocity means moving to the right
  • Negative velocity means moving to the left
  • Flat (constant) velocity just means velocity isn’t changing (constant motion)
    • Does NOT mean it isn’t moving
    • Zero velocity means it isn’t moving
  • BIG THING: pay close attention to the sign (positive or negative) of the velocity
    • Tells you direction

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slide4
Consider the following velocity vs. time graph. Write down a description of the person’s motion with as much detail as possible.At the end of the motion, the person’s displacement is:
  • Positive
  • Negative
  • Not enough information
  • None of the above
average acceleration
Average Acceleration
  • We need to keep track not only of the fact that something is moving but how that motion is changing.
  • Define the average acceleration by

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technical term alert
Technical term alert!
  • Note that in physics we use the term “acceleration” in a technically defined way:
    • “acceleration” = changing velocity
  • The object may be speeding up or slowing down or keeping the same speed and changing direction. We still say “it is accelerating.”
  • In common speech “acceleration” = speeding up,“deceleration” = slowing down, and “turning”=changing direction.
  • How many (physics) accelerators are there on your car?

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average acceleration1
Average Acceleration

Note!

Figure 2.9

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uniformly changing motion
Uniformly changing motion
  • If an object moves so that it changes its velocity by the same amount in each unit of time, we say it is in uniformly accelerated motion.
  • This means the average acceleration will be the same no matter what interval of time we choose.

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instantaneous acceleration
Instantaneous acceleration
  • Sometimes (often) an object will move so that sometimes it speeds up or slows down at different rates.
  • We want to be able to describe this change in motion also.
  • If we consider small enough time intervals, the change in velocity will look uniform — for a little while at least.

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relationship between acceleration and velocity
Relationship Between Acceleration and Velocity
  • Uniform velocity (shown by red arrows maintaining the same size)
  • Acceleration equals zero

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relationship between velocity and acceleration
Relationship Between Velocity and Acceleration
  • Velocity and acceleration are in the same direction
  • Acceleration is uniform (blue arrows maintain the same length)
  • Velocity is increasing (red arrows are getting longer)
  • Positive velocity and positive acceleration

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relationship between velocity and acceleration1
Relationship Between Velocity and Acceleration
  • Acceleration and velocity are in opposite directions
  • Acceleration is uniform (blue arrows maintain the same length)
  • Velocity is decreasing (red arrows are getting shorter)
  • Velocity is positive and acceleration is negative

University of Maryland

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