Physics 121: Fundamentals of Physics I

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# Physics 121: Fundamentals of Physics I - PowerPoint PPT Presentation

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

September 13, 2006

University of Maryland

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

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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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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
• 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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• 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 Acceleration

Note!

Figure 2.9

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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
• 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
• Uniform velocity (shown by red arrows maintaining the same size)
• Acceleration equals zero

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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 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

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