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Physics 121: Fundamentals of Physics IPowerPoint Presentation

Physics 121: Fundamentals of Physics I

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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

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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

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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 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

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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?

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

Note!

Figure 2.9

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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.

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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.

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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

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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

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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

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