Q.

A small ball of mass M and density ρ is dropped in a viscous liquid of density ρ0. After some time, the ball falls with a constant velocity. What is the viscous force on the ball           [2023]

1 F=Mg(1-ρ0ρ)  
2 F=Mg(1+ρρ0)  
3 F=Mg(1+ρρ0)  
4 F=Mg(1+ρ0ρ)  

Ans.

(1)

For constant velocity Fnet=0

Fvis+ρ0vg=ρvg

Fvis=(ρ-ρ0)vg

=ρvg(1-ρ0ρ)=Mg(1-ρ0ρ)