Question

It is a common observation that rain clouds can be at about 1 km altitude above the ground. If a rain drop falls from such a height freely under gravity, then what will be its speed in km h^-1?(Take g = 10 m s^-2)

• A. 510
• B. 610
• C. 710
• D. 910

Answer 1

Answer: (A)

510

Answer 2

Here , u = 0, g = $10 \mathrm {~ms} ^ { - 2 }$, h = 1 km = 1000m

As

$\therefore \mathrm { v } ^ { 2 } = 2 \mathrm { gh }$

or $\mathrm { v } = \sqrt { 2 \mathrm { gh } } = \sqrt { 2 \times 10 \times 1000 } = 100 \sqrt { 2 } \mathrm {~ms} ^ { - 1 }$ $= 100 \sqrt { 2 } \times \frac { 18 } { 5 } \mathrm { kmh } ^ { - 1 }$

$= 360 \sqrt { 2 } \mathrm { kmh } ^ { - 1 } \approx 510 \mathrm { kmh } ^ { - 1 }$