#### The amplitude of a simple pendulum,  oscillating in air with a small spherical bob,   decreases from 10 cm to 8 cm in 40 seconds.Assuming that Stokes law is valid, and ratio of the coefficient of viscosity of air to that of carbon dioxide is 1.3, the time in which amplitude of this pendulum will  reduce  from  10  cm  to  5  cm  in  carbondioxide will be close to     Option 1) 231 s Option 2) 208 s Option 3) 161 s Option 4) 142 s

As we discussed in

As we know that

From question

Similarly

Solving equations (i) and (ii) we get

Option 1)

231 s

This option is incorrect.

Option 2)

208 s

This option is incorrect.

Option 3)

161 s

This option is incorrect.

Option 4)

142 s

This option is correct.