1 of 24© Boardworks Ltd 2008. 2 of 24© Boardworks Ltd 2008.

1 of 24 © Boardworks Ltd 2008

2 of 24 © Boardworks Ltd 2008

3 of 24 © Boardworks Ltd 2008

To work out the speed of an object you need to know:

Distance, time and speed

the distance travelled how long it took to travel that distance.

4 of 24 © Boardworks Ltd 2008

Average speed is calculated using this equation:

Speed can be measured in different units, e.g. m/s, km/h, km/s, miles per hour.

The units of distance and time will determine the units to be used for speed.

Calculating average speed

d

s t

formula triangle

total distance

total time

Averagespeed

=

5 of 24 © Boardworks Ltd 2008

Words and units

6 of 24 © Boardworks Ltd 2008

Speed formula triangle

7 of 24 © Boardworks Ltd 2008

A girl takes 30 minutes to travel around a cross-country ski course: a distance of 10 km. Calculate her average speed in km/h.

20 km/h

Speed calculation example

distance (in km)average speed (in km/h) time (in h)

=

10 km

0.5 h=

10 km=

8 of 24 © Boardworks Ltd 2008

Sometimes the units have to be changed in a speed calculation.

Speed calculation example – units

Calculate the average speed of the skier in m/s.

10 km

2.8 m/s

average speed (in m/s) time (in s)

=distance (in m)

3600 s10,000 m

= =

9 of 24 © Boardworks Ltd 2008

= 3.6 km/h × 0.5 h

distance (km) = speed (km/h) × time (h)

A boy sets off for a walk around his village at an average speed of 3.6 km/h. How far will he travel in 30 minutes?

Give your answer in km.

Distance calculation

= 1.8 km

10 of 24 © Boardworks Ltd 2008

How long would it take the boy to walk 200 m to the church, if his average speed was 0.8 m/s?

Time calculation – question 1

time =distance

speed

= 250 seconds

200 m

0.8 m/s =

11 of 24 © Boardworks Ltd 2008

12 of 24 © Boardworks Ltd 2008

Car graphing activity – animation

13 of 24 © Boardworks Ltd 2008

Distance–time graph

14 of 24 © Boardworks Ltd 2008

The gradient of the line in adistance–time graph equals the speed.

Calculating speed from the gradient

The slope of a graph is called the gradient.

It is difficult to calculate the gradient of ‘realistic’ graphs because the line is curved.

Simple graphs use straight lines only, making it easy to calculate the gradient.

time

dis

tan

ce

time

dis

tan

ce

15 of 24 © Boardworks Ltd 2008

What is the speed of the object between points A and B?

time (s)

dis

tan

ce (

m)

0 21 3 4 5 6 7 8 90

10

20

30

40

50

60

70

A

B the object has moved

60 m (70 – 10 )

it took 3 s to move this distance (6 – 3)

speed = distance/time

= 60/3

= 20 m/s

What’s the speed?

16 of 24 © Boardworks Ltd 2008

Speed–time graph

17 of 24 © Boardworks Ltd 2008

18 of 24 © Boardworks Ltd 2008

How can we work out the average speed of this object between points C and D?

time (s)

dis

tan

ce (

m)

0 21 3 4 5 6 7 8 90

10

20

30

40

50

60

70

A

B the object has moved

70 m (70 – 0 )

it took 9 s to move this distance (9 – 0)

speed = distance/time

= 70/9

= 7.8 m/s

What’s the speed?

C

D

19 of 24 © Boardworks Ltd 2008

The speed of a race car at any point around a track is called the instantaneous speed. A car will have a different instantaneous speed at each of the points around the track.

Average speed is the total distance travelled divided by the total time taken. It is never bigger than the instantaneous speed.

Average speed vs. instantaneous speed

20 of 24 © Boardworks Ltd 2008

Calculating average speeds

21 of 24 © Boardworks Ltd 2008

22 of 24 © Boardworks Ltd 2008

Glossary

23 of 24 © Boardworks Ltd 2008

Anagrams

24 of 24 © Boardworks Ltd 2008

Multiple-choice quiz