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Words and units

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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.

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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

Average

speed=

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A girl takes 30 minutes to travel around a cross-country ski

course: a distance of 10km. Calculate her average speed

in km/h.

20km/h

Speed calculation example

distance (in km)average speed

(in km/h) time (in h)=

10 km

0.5h=

10km=

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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

2.8m/s

average speed

(in m/s) time (in s)=

distance (in m)

3600 s

10,000m= =

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= 3.6km/h × 0.5h

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

= 1.8km

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How long would it take the boy to walk 200m to the church, if

his average speed was 0.8m/s?

Time calculation – question 1

time =distance

speed

= 250 seconds

200m

0.8m/s=

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Distance–time graph

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The gradient of the line in a

distance–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

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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

60m (70 – 10)

it took 3s to move this

distance (6 – 3)

speed = distance/time

= 60/3

= 20m/s

What’s the speed?

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Speed–time graph

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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

70m (70 – 0)

it took 9s to move this

distance (9 – 0)

speed = distance/time

= 70/9

= 7.8m/s

What’s the speed?

C

D

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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