# AVERAGE SPEED AND AVERAGE VELOCITY

It's very unusual for something to travel at an exactly constant speed - it will probably change. Imagine that you are in a bus travelling along a motorway to London. Lets say that it is 300 km and that it takes 5 hours. This does not mean that it has been going at exactly 60 km/hour all the time. Sometimes it will have been going faster and sometimes slower.

However the AVERAGE speed of the bus will be 60 km/hr.

AVERAGE SPEED = TOTAL DISTANCE/TOTAL TIME
TOTAL DISTANCE = AVERAGE SPEED x TIME

If you are in car that changes its speed STEADILY from one speed to another then:

AVERAGE SPEED = [FINAL SPEED – INITIAL SPEED]/2

The same also applies to the velocity. You can work out the average velocity of something in just the same way.

If you are in car that changes its velocity STEADILY from one velocity to another then:

AVERAGE VELOCITY = [FINAL VELOCITY - INITIAL VELOCITY]/2

STUDENT INVESTIGATIONS
Work out the average velocities of some members of your class:
(a) on their way to school
(b) during a Physics lesson

PROBLEMS
1. If the changes in speed are steady work out the average velocities for:
(a) a car going from 30m/s to 50m/s
(b) a cyclist going from 10m/s to 4/s
(c) a train going from 0m/s to 50m/s
(d) a bus going from 12m/s to 9m/s

2. Work out the distances travelled by the vehicles in question one if they change their speed in :
(a) 15s   (b) 3 mins   (c) 20 mins   (d) one hour

© Keith Gibbs 2011