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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

Physics 121:Fundamentals of Physics I

September 13, 2006

University of Maryland


Interpreting position vs time graphs
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


Interpreting velocity vs time graphs
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


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
Average Acceleration a description of the person’s motion with as much detail as possible.

  • 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


Technical term alert
Technical term alert! a description of the person’s motion with as much detail as possible.

  • 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


Average acceleration1
Average Acceleration a description of the person’s motion with as much detail as possible.

Note!

Figure 2.9

University of Maryland


Uniformly changing motion
Uniformly changing motion a description of the person’s motion with as much detail as possible.

  • 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


Instantaneous acceleration
Instantaneous acceleration a description of the person’s motion with as much detail as possible.

  • 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


Relationship between acceleration and velocity
Relationship Between Acceleration and Velocity a description of the person’s motion with as much detail as possible.

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

  • Acceleration equals zero

University of Maryland


Relationship between velocity and acceleration
Relationship Between Velocity and Acceleration a description of the person’s motion with as much detail as possible.

  • 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


Relationship between velocity and acceleration1
Relationship Between Velocity and Acceleration a description of the person’s motion with as much detail as possible.

  • 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


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