The critical velocity of an oil drop in air is . What is the radius of the drop? If two such drops coalesce than what will be the terminal velocity of the resultant drop. Coefficient of viscosity of oil is  and density is . The density of air in comparison to oil is negligible and .


Correct option is


By Stokes’ law, the terminal velocity of oil drop of radius r in air is given by 


where  is the density of oil,  is the density of air and  is the coefficient of viscosity of air. Here  is negligible. 

Substituting the values



Now, suppose on coalescing of two drops, the radius of the resultant drop is R. since the total volume is fixed, we have


If the critical velocity of the resultant drop be v’, then from eq. (i), we have





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