Physics 121: Fundamentals of Physics I

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

2. 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

3. 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 University of Maryland

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: • Positive • Negative • Not enough information • None of the above

5. 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 University of Maryland

6. 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? University of Maryland

7. Average Acceleration Note! Figure 2.9 University of Maryland

8. 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. University of Maryland

9. 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. University of Maryland

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

11. 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 University of Maryland

12. 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