Question

A tube of uniform cross section always has water flowing through it. It is kept vertical in such a way that water enters from top and leaves from the bottom. If the speed at a point A below the opening is ‘v’, what will be the speed at a point B vertically below A such that the distance between A & B is ‘2h’?

(a) v

(b) \(\sqrt{v^2 + 4gh}\)

(c) \(\sqrt{v^2 + 2gh}\)

(d) v/2

This question was addressed to me during an interview.

The query is from Fluids Mechanical Properties in section Mechanical Properties of Fluids of Physics – Class 11

Answer 1

Correct answer is (a) v

The explanation is: According to the equation of continuity, the velocity at B will be the same as that at A since the area of the cross section is the same at both points.                                                                                              You may be confused by thinking that gravity will increase its velocity but the pressure will also decrease downwards and therefore net effect will be zero change in velocity.