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Position-Time Graphs Velocity-Time Graphs

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Position-Time GraphsVelocity-Time Graphs

Using graphs to describe motion

- To begin, consider a car moving with a constant, rightward (+) velocity.
- Calculate the velocity of the car in the diagram below. (v = d / t)
- v= 10m/s

- A review:
- Axis
- The variables
- Point of Origin

- What does the slope signify?
- Slope = velocity
- What is the slope of this line?

- What would happen if I had a horizontal line, what does that mean?
- What about a vertical line, what would that mean?

- The object is changing its position in the north direction from 0 to 3 seconds, the velocity is uniform.
- The object stays at the same position for about 3 seconds - its stopped for 3 seconds.
- From the 6th to the 8th second the object is backtracking, its returning to the starting position ( our reference point ).
- From the 8th second on the object passes the starting position and travels southward from it.

- Consider a car moving with a constant, rightward (+) velocity of +10 m/s.
- A car moving with a constant velocity is a car moving with zero acceleration

- What can you tell me about this graph?
- What does the slope of this graph represent?

- Since the graph is a velocity-time graph, the velocity is positive whenever the line lies in the positive region
- Similarly, the velocity is negative whenever the line lies in the negative region
- a positive velocity means the object is moving in the positive direction; and a negative velocity means the object is moving in the negative direction.
- So if an object is moving in the positive direction, the line is located in the positive region of the velocity-time graph (regardless if it is sloping up or sloping down).

- Finally, if a line crosses the x-axis from the positive region to the negative region of the graph (or vice versa), then the object has changed directions.

- Our data is captured when the object is already moving, it has a constant velocity of 30m/s [N] up to the 3rd second of observation.
- The object decelerates, reaches no velocity and then begins traveling in the south direction.
- The object travels at a constant velocity south for 3 seconds
- The object smoothly decelerates and comes to a stop