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Physics 121: Fundamentals of Physics I

Physics 121: Fundamentals of Physics I. September 11, 2006. From Last Time…. Displacement = Change in Position Displacement is a vector, direction (or sign in 1D) is important Distance is not a vector Like car odometer Average Velocity = Displacement divided by Time Taken

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Physics 121: Fundamentals of Physics I

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  1. Physics 121:Fundamentals of Physics I September 11, 2006 University of Maryland

  2. From Last Time… • Displacement = Change in Position • Displacement is a vector, direction (or sign in 1D) is important • Distance is not a vector • Like car odometer • Average Velocity = Displacement divided by Time Taken • Velocity is a vector, direction (or sign in 1D) is important • Speed is not a vector University of Maryland

  3. Graphing Position • Describe where something is in terms of its coordinate at a given time. University of Maryland

  4. Uniform motion • If an object moves so that it changes its position by the same amount in each unit of time, we say it is in uniform motion. • This means the average velocity will be the same no matter what interval of time we choose. University of Maryland

  5. After it is wound and released, a wind-up car travels at almost a constant velocity. Assuming it takes a negligible time to get up to speed, what does the graph of its velocity look like as a function of time? • Graph One • Graph Two • Graph Three • Graph Four • Graph Five • None of the above

  6. Graphing Velocity • An object in uniform motion has constant velocity. • This means the instantaneous velocity does not change with time. Its graph is a horizontal line. University of Maryland

  7. Dx Dt Graphing velocity: From the position graph • You can also figure out the velocity graph from the position graph using Slope = <v> University of Maryland

  8. Average Velocity, Non Constant • The motion is non-constant velocity • The average velocity is the slope of the blue line joining two points • Instantaneous velocity is slope if tangent line (green lines) at a point Dx Dt University of Maryland

  9. 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 University of Maryland

  10. Q R P position(arbitrary units) 0 1 2 3 4 A person initially at point P in the illustration stays there a moment and the moves along the axis to Q and stays there a moment. She then runs quickly to R, stays there a moment, and then strolls slowly back to P. Which of the position vs. time graphs below correctly represents the motion? • Graph One • Graph Two • Choice Three • Graph Four • Choice Five • Graph Six

  11. 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

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