Question

Q1. A car travels at 72 km/h for first 30 s, 18km/h for next 40 s and finally 9 km/h for next
10 s. Find its average speed.

Answer 1

Explanation:

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Answer 2

v(av) = (Total distance)/(Total time)

∵ s = vt

⇒ v(av) = (vt + v′t′ + v″t″)/(t + t′ + t″)

⇒ v(av) = (72 km/h × 30 s + 18 km/h × 40 s + 9 km/h × 10 s)/(30 s + 40 s + 10 s)

⇒ v(av) = (20 m/s × 30 s + 5 m/s × 40 s + 2.5 m/s × 10 s)/(80 s)

⇒ v(av) = (600 m + 200 m + 25 m)/(80 s)

⇒ v(av) = (825 m)/(80 s)

⇒ v(av) = 10.3125 m/s.

∴ Average speed of the whole journey is approximately equal to 10.3 m/s.

More:

Shortcut method to tackle such questions:

If a, b, and c are the 3 speed values given. Then put those values in the below formula in order to get the average speed:-

3abc/(ab + bc + ac).