1. Velocity-time Graphs:
1.3
Velocity-time Graphs:
Uniform and Non-Uniform Motion

2. The airplane is flying with a constant velocity of 900 km/h east.
Time (h)
Velocity (km/h) [E]
0.0
900
1.0
2.0
3.0
4.0
5.0
The airplane is flying with a constant velocity of 900 km/h east.
The shape of the velocity-time graph is a horizontal line.
This indicates that the plane exhibits uniform motion.

3. slope =
The slope of the line on a velocity-time graph is the acceleration of the object.
uniform motion
3:34
accelerated motion

4. Metres per second squared??
(m/s)
“metres per second per second”
(s)
For example, an object with an acceleration of +2.0 m/s2 increases its velocity by 2.0 m/s each second.
Acceleration can be described as a rate of change of velocity.
So, “metres per second squared” is the same as “metres per second per second.”

5. Position (m) [forward]
Non-uniform Motion
The dragster is increasing it’s speed.
An object that is accelerating (changing speed and/or direction) is exhibiting non-uniform motion.
Time (s)
Position (m) [forward]
0.0
1.0
2.0
8.0
3.0
18.0
4.0
32.0
5.0
50.0

6. On a position-time graph, the slope is the object’s velocity.
A curved line has a changing slope.
A curved line on a position-time graph means the object is accelerating.

7. If you see either one of these curves, then it is positive acceleration.
If you see either one of these curves, then it is negative acceleration.

8. Instantaneous Velocity
Instantaneous velocity is an object’s velocity at a specific instant of time.
A tangent is a straight line that touches the curve only at one point.
The velocity of the dragster at 2.0 s is given by the slope of the tangent.
The slope of the tangent at 2.0 s is 7.0 m/s [forward].

9. The slope of the tangent at 3.0 s is 13 m/s [forward].
Any object with constant acceleration is undergoing uniformly accelerated motion.

10. Velocity (m/s) [forward]
Time (s)
Velocity (m/s) [forward]
0.0
2.0
7.0
3.0 13
4.0 16
A line with a constant slope on a velocity-time graph represents uniformly accelerated motion.
The object in this case is speeding up.

11. Acceleration vs Time Graphs
A constant slope on a velocity-time graph means a constant rate of acceleration.
This is shown as a horizontal line on an acceleration-time graph.

12. The ATV in example 1.5 is slowing down.
The magnitude of the velocity is decreasing at a constant rate.

13. Negative Acceleration Does Not Necessarily Mean Slowing Down
When the directions (signs) of velocity and acceleration are the same (positive or negative), the object is speeding up.
When velocity and acceleration have opposite directions (signs), the object slows down.
Think about the velocity and acceleration vectors when we throw a ball straight up into the air.
Remember, we use positive and negative signs in physics to indicate direction!
Professor Frink sez:
1:58

14. Homework:
read pages 21 – 29
1.3 Check and Reflect
pg. 30 #’s 1 – 3