A hot air balloon of radius 10m is floating at a constant altitude over Parliament House in Canberra. It is initially at rest. Suddenly, an easterly breeze of speed 10 cm s-1 starts blowing. Assuming that Stokes' law applies, how much force does the wind apply to the balloon? The viscosity of air is 1.8x10-5 Pa s.
A) 3.4x10-5 N
B) 3.4x10-4 N
C) 3.4x10-3 N
D) 3.4x10-2 N
E) 3.4x10-1 N
ANS

Question

Quizplus · Accepted Answer

The drag force on the balloon is given by Stokes' law:$$F_d=6\pi\eta rv$$where $\eta$ is the viscosity of air, $r$ is the radius of the balloon, and $v$ is the speed of the wind.Plugging in the given values, we get:$$F_d=6\pi(1.8	imes10^{-5}	ext{ Pa s})(10	ext{ m})(0.1	ext{ m/s})=3.4	imes10^{-4}	ext{ N}$$

A Hot Air Balloon of Radius 10m Is Floating at a Constant