Speed, velocity and acceleration

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)

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

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.

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

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)

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

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

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.

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

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

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

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

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