Q.

The viscous drag acting on a metal sphere of diameter 1 mm, falling through a fluid of viscosity 0.8 Pa s with a velocity of 2 ms-1 is equal to           [2023]
 

1 15×10-3N  
2 30×10-3N  
3 1.5×10-3N  
4 20×10-3N  

Ans.

(1)

Stoke's law relates the backward dragging force F acting on a small sphere of radius r moving through a viscous medium of viscosity η with velocity v

F=6πrηv

=(6)(3.14)(1×10-32)(0.8)(2)=15×10-3 N