class 9 describing motion

6. Steady Speed…
• If something is moving at a steady speed, it
means we expect the same increase in
distance in a given time:
• Time is increasing to the right, and
distance is increasing steadily with time. It
moves at a steady speed.

8. Distance-time graphs
40
30
20
10
0
20 40 60 80 100
4) Diagonal line
downwards =
3) Steeper diagonal line =1) Diagonal line =
2) Horizontal line =
Distance
(metres)
Time/s

9. 40
30
20
10
0
20 40 60 80 100
1) What is the speed during the first 20 seconds?
2) How far is the object from the start after 60 seconds?
3) What is the speed during the last 40 seconds?
4) When was the object travelling the fastest?
Distance
(metres)
Time/s

10. Speed vs. Velocity
Speed is simply how fast you are travelling…
Velocity is “speed in a given direction”…
This car is travelling at a speed of 20m/s
This car is travelling at a velocity of 20m/s
east

11. Acceleration occurs when:
• Velocity increases
• Velocity decreases
• Velocity Changes in direction
Acceleration is the rate of change in velocity. Is a vector quantity measured in m/s2
a = acceleration (m/s2)
ΔV = change in velocity (m/s)
Δt = change in time (s)

15. When does the object move with constant acceleration?
When does the object move with constant velocity?
When is the object at rest?
What is the greatest speed reached by the object?
Remember:
The area under a graph
of speed versus time
represents the distance

17. Be careful!
distance
time
speed
time

18. Constant speed
distance
time
speed
time
Area under graph = distance
travelled
gradient = speed