D. Roberts PHYS 121 University of Maryland Physics 121: Fundamentals of Physics I September 11, 2006

D. Roberts PHYS 121 University of Maryland 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

D. Roberts PHYS 121 University of Maryland Graphing Position Describe where something is in terms of its coordinate at a given time.

D. Roberts PHYS 121 University of Maryland 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.

D. Roberts PHYS 121 University of Maryland Example 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 2.Graph Two 3.Graph Three 4.Graph Four 5.Graph Five 6.None of the above

D. Roberts PHYS 121 University of Maryland 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.

D. Roberts PHYS 121 University of Maryland Graphing velocity: From the position graph You can also figure out the velocity graph from the position graph using xx tt Slope =

D. Roberts PHYS 121 University of Maryland 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 tt xx

D. Roberts PHYS 121 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,  x/  t, not a ratio of coordinates (not x/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

D. Roberts PHYS 121 University of Maryland Example 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? position (arbitrary units) QRP

? 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 2.Graph Two 3.Choice Three 4.Graph Four 5.Choice Five 6.Graph Six position (arbitrary units) QRP

? Example 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: 1.Positive 2.Negative 3.Not enough information 4.None of the above

? 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 2.Negative 3.Not enough information 4.None of the above