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# Position-Time Graphs Velocity-Time Graphs PowerPoint PPT Presentation

Position-Time Graphs Velocity-Time Graphs. Using graphs to describe motion. The line. 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. The Graph. A review: Axis The variables

Position-Time Graphs Velocity-Time Graphs

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## Position-Time GraphsVelocity-Time Graphs

Using graphs to describe motion

### The line

• 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

### The Graph

• A review:

• Axis

• The variables

• Point of Origin

### The Slope of a P/T graph

• What does the slope signify?

• Slope = velocity

• What is the slope of this line?

### What is going on here?

• What would happen if I had a horizontal line, what does that mean?

• What about a vertical line, what would that mean?

### You should be able to do:

• 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.

### Acceleration

• 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

### Velocity-Time Graph

• What does the slope of this graph represent?

### Acceleration cont.

• 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).

### Acceleration cont.

• 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.

### Example of a difficult v-t Graph

• 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