Question

Spherical balls of radius R are falling in a viscous fluid of viscosity Ƞ with a velocity v. The retarding viscous force acting on the spherical ball is ___________

(a) Directly proportional to R but inversely proportional to v

(b) Directly proportional to both radius R and velocity v

(c) Inversely proportional to both radius R and velocity v

(d) Inversely proportional to R but directly proportional to velocity v

The question was posed to me in an interview for job.

My question is from Buoyancy topic in section Mechanical Properties of Solids of Engineering Physics I

Answer 1

Correct choice is (b) Directly proportional to both radius R and velocity v

Easiest explanation: F = 6πȠRv, which is retarding viscous force is directly proportional to both R and v.