Unit 18 Section 1 : Speed

In this section we introduce the idea of speed, considering both instantaneous speed and average speed.
Instantaneous speed = speed at any instant in time

Average speed =
distance travelled

time taken

If a car travels 100 miles in 2 hours:

Average speed =
100 miles

2 hours
= 50 miles per hour (mph)
The car does not travel at a constant speed of 50 mph; its speed varies during the journey between 0 mph and 70 mph.
The speed at any particular time is called the instantaneous speed.

The following table lists units in common use for speed and their abbreviations:

Example Questions

Example Question 1
Judith drives from Plymouth to Southampton, a distance of 160 miles, in 4 hours.
She then drives from Southampton to London, a distance of 90 miles, in 1 hour and 30 minutes.

Determine her average speed for each journey.

For the Plymouth to Southampton journey, we divide 160 miles by 4 hours.
Her average speed on this part of the journey was 40 mph.

For the Southampton to London journey, we divide 90 miles by 1.5 hours.
Her average speed on this part of the journey was 60 mph.

Example Question 2
John can type 960 words in 20 minutes.

(a) Calculate his average typing speed in words per minute.
To work this out, we divide 960 words by 20 minutes.
His average typing speed is 48 words per minute (wpm).

(b) Calculate his average typing speed in words per hour.
If he can type 48 words per minute, then he can type 60 times as many in an hour.
His average typing speed is 2880 words per hour (because 60 × 48 = 2880).

Practice Question

Work out the answer to these questions then click on the button marked

to see whether you are correct.

A marathon runner completed 24 miles in 4 hours.

(a) What was their average speed in miles per hour?

(b) What was their average speed in miles per minute?

Exercises

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You have now completed Unit 18 Section 1

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Produced by A.J.Reynolds August 2008