Distance, time and speed

1 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.


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

=


Words and units


Speed formula triangle


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=


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

= =


= 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


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 =



Car graphing activity – animation


Distance–time graph


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


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?


Speed–time graph



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


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


Calculating average speeds



Glossary


Anagrams


Multiple-choice quiz

Distance, time and speed

Documents