CLASS 9 SCIENCE · MOTION

Speed, distance and time

Average speed = total distance ÷ total time. To change km/h to m/s multiply by 5/18; m/s to km/h multiply by 18/5.

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Worked examples

Example 1 · level 1

Diya cycles at a steady 12 m/s. How many seconds does it take to cover 240 m?

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time = distance ÷ speed
= 240 ÷ 12 = 20 s.

Answer: 20 s

Example 2 · level 2

A cheetah in a wildlife documentary runs at 5 m/s. Express this speed in km/h.

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5 m/s = 5 × 18/5 km/h
= 18 km/h.

Answer: 18

Example 3 · level 3

Rohan drives from Pune at 20 km/h for 2 h and then at 30 km/h for 2 h. Find the average speed for the whole journey in km/h (correct to 2 decimal places).

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Total distance = 40 + 60 = 100 km.
Total time = 2 + 2 = 4 h.
Average speed = 100 ÷ 4 = 25 km/h.

Answer: 25