Verified Solution • 2025–26 Session moderate

QJEEM21Ph1fcb: JEE Main 2021 Physics - Class 11 JEEM21Ph1fcb Solution: A raindrop with radius R = 0.2 mm falls from a cloud at a…

A raindrop with radius R = 0.2 mm falls from a cloud at a height h = 2000 m above the ground. Assume that the drop is spherical throughout its fall and the force of buoyance may be neglected, then the terminal speed attained by the raindrop is :

[Density of water fw = 1000 kg m −- 3 and Density of air fa = 1.2 kg m −- 3, g = 10 m/s2, Coefficient of viscosity of air = 1.8 ×\times 10 −- 5 Nsm −- 2]

Multiple Choice Options

  • A 250.6 ms −- 1
  • B 43.56 ms −- 1
  • C 4.94 ms −- 1
  • D 14.4 ms −- 1

Step-by-Step Verified Solution

At terminal speed

a = 0

Fnet = 0

mg = Fv = 6 π\pi η\eta Rv

v=mg6πηRvv = {{mg} \over {6\pi \eta Rv}}

v=ρw4π3R3g6πηRv = {{{\rho _w}{{4\pi } \over 3}{R^3}g} \over {6\pi \eta R}}

=2ρwR2g9η= {{2{\rho _w}{R^2}g} \over {9\eta }}

=40081= {{400} \over {81}} m/s

= 4.94 m/s

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