1. D. Roberts PHYS 121 University of Maryland Physics 121: Fundamentals of Physics I September 13, 2006

2. 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 –Positive position means to right of origin –Negative position means to left of origin

3. D. Roberts PHYS 121 University of Maryland 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

4. ? 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: 12345 1.Positive 2.Negative 3.Not enough information 4.None of the above

5. D. Roberts PHYS 121 University of Maryland 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

6. D. Roberts PHYS 121 University of Maryland 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?

7. D. Roberts PHYS 121 University of Maryland Average Acceleration Figure 2.9 Note!

8. D. Roberts PHYS 121 University of Maryland 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.

9. D. Roberts PHYS 121 University of Maryland 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.

10. D. Roberts PHYS 121 University of Maryland Relationship Between Acceleration and Velocity Uniform velocity (shown by red arrows maintaining the same size) Acceleration equals zero

11. D. Roberts PHYS 121 University of Maryland 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

12. D. Roberts PHYS 121 University of Maryland 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