Page 128 - ICSE Math 7

11 Speed, Distance and Time

Key Concepts

• Speed and Units of Speed • Average Speed
• Conversion of Units of Speed • Relative Speed

In this chapter, we will learn about speed and conversion of units of speed. We will also learn about
relative speed and how to solve problems based on speed, time and distance.

Speed

When we walk or run, the change in time is directly proportional to Maths Info
the change in distance covered.
The distance covered by a body in unit time is called its speed. • Change in speed is directly
proportional to change in
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For example, if a car covers 80 km in 2 hours, then it will cover distance covered.
2 Distance = Speed × Time
km, i.e., 40 km in 1 hour. So, its speed is 40 kilometres per hour.
• Change in speed is inversely
Distance covered proportional to change in
Thus, Speed = . If a body covers an equal distance
Time taken time taken.
in equal intervals of time, then its speed is said to be uniform otherwise Time = Distance
Speed
its speed is variable or non uniform.

Units of speed

If the distance covered is in kilometres (km) and the time taken is in hours (h), then the unit of speed
–1
is kilometre(s) per hour or km h . If the distance covered is in centimetres (cm) and the time taken
–1
is in seconds (s), then the unit of speed is centimetre(s) per second or cm s .

Conversion of units of speed
–1
(a) To convert km h into m s –1
1 km = 1,000 m and 1 h = (60 × 60) s = 3,600 s
1,000 m 5
–1
\ 1 km h = = m s –1
3,600 s 18
–1
–1
Thus, to convert km h into m s , multiply by 5 .
18
–1
–1
Conversely, to convert m s into km h , multiply by 18 .
5
–1
–1
(b) To convert km h into cm s .
1 km = 1,000 m = 1,000 × 100 cm = 1,00,000 cm and 1 h = (60 × 60) s = 3,600 s
1,00,000 cm 250
–1
\ 1 km h = = cm s –1
3,600 s 9
–1
–1
Thus, to convert km h into cm s , multiply by 250 .
9
–1
–1
Conversely, to convert cm s into km h , multiply by 9 .
250
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