1. Speed, velocity and acceleration

2. Speed = distance travelled per unit of time
How can we describe how fast an object moves?
Speed is a measure of how fast something moves.
Speed = distance travelled per unit of time
SI unit: m s–1
or km h–1 (for long distances)

3. a Average speed A car travels at 50 km h–1, slows down to 0 km h–1,
and speeds up again to 60 km h–1.
Its average speed over the whole journey
overall distance travelled =
total time of travel

4. 1 Speed
a Average speed
Average speed does not tell the variations during the journey.
On most trips, the speed at any instant is often different from the average speed.

5. Speedometer tells the car’s speed at any instant!
b Instantaneous speed
Speedometer tells the car’s speed at any instant!

6. rate of change of displacement.
2 Velocity
Velocity is...
a speed in a given direction or
rate of change of displacement.
direction
a vector
quantity
velocity
magnitude
(speed)

7. a Speed with direction MTR drivers concern speed only.
2 Velocity
a Speed with direction
MTR drivers concern speed only.
speed = 90 km h–1
Pilots concern velocity (direction & speed).
speed = 300 km h–1
direction = west

8. b Average velocity overall displacement Average velocity =
total time of travel
direction of overall displacement
direction of velocity =

9. c Instantaneous velocity
The velocity at any instant is called instantaneous velocity.
If a car moves at a constant velocity...
… its average and instantaneous velocities have the same value.

10. 3 Acceleration When a car moves faster and faster,
its speed is increasing (velocity changed).

11. 3 Acceleration When a car moves slower and slower,
its speed is decreasing (velocity changed).

12. 3 Acceleration
When a car changes direction,
its velocity changes too.

13. 3 Acceleration Acceleration measures the change in velocity direction
speed
Acceleration = velocity per unit time
overall change in velocity =
total time taken
vector quantity
Unit: m s–1 / s
= m s–2

14. Q2 When time is measured... Unit of time: hour (h)
Unit of distance/displacement: kilometer (km)
Quantity Unit Scalar/Vector
Speed ______ _____
Velocity ______ _____
Change in velocity ______ _____
Acceleration ______ _____
km h–1
scalar
km h–1
vector
km h–1
vector
km h–2
vector