question_answer

Spherical balls of radius R are falling g in a viscous fluid of viscosity. The retarding viscous force acting on the spherical ball is                                                                                                            [AIEEE 2004]

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. Correct Answer: B Solution : [b] Retarding force acting on a ball falling into a viscous fluid

where,   R = radius of ball,

v = velocity of ball,

and       coefficient of viscosity.

and

Or in words, retarding force is directly proportional to both R and v.