Apply the concept formula to get the answer
When a sphere of radius r moves with a velocity v through a fluid of viscosity η, the viscous force opposing the motion of the sphere is
F = 6πη r v
If for a sphere viscous force become equal to the net weight acting downward, the velocity of the body become constant and is known as terminal velocity.
6πη r vT = (4/3) πr3 (ρ – σ)g