Question

On which these factors does air/fluid resistance force, acting on a spherical body, not depend?

(a) Radius of body

(b) Velocity of body

(c) Coefficient of viscosity

(d) Density of body

This question was posed to me in a job interview.

This is a very interesting question from Fluids Mechanical Properties in section Mechanical Properties of Fluids of Physics – Class 11

Answer 1

Right option is (d) Density of body

The best explanation: Resistance force by a fluid on a body = 6πηrv, where η is coefficient of viscosity, r is radius of body & v is its velocity. This doesn’t depend on density.

Another way to think of this is to imagine a hollow and solid sphere of the same radius falling through air at the same speed. The resistance offered by air will be the same on both as they have the same surface area exposed and are moving at the same speed.