Question

The deeper we go inside a flowing river, the fluid friction:
A) decreases
B) increases
C) remains same
D) becomes zero

Answer 1

Hint: In this question use the concept as you go deeper into a flowing river, you spend more energy as you try to move forward. The speed of swimming decreases. So, the friction increases the deeper we go in a flowing river.

Complete step by step solution:
As we know that the friction is the type of force which resists the motion of any two solid surfaces or fluid layers. When this friction is experienced inside any fluid then it is called fluid friction or fluid viscosity.
Fluid friction is described as the friction between any two viscous layers of a fluid that are moving with respect to each other. The fluid friction depends upon the viscosity of the fluid and the velocity gradient. The velocity gradient is the rate of change of velocity or simply the ratio of velocity and distance between layers.
As we go deeper inside a flowing river, we require more force to keep the layers flowing with flow.
$ \Rightarrow F = \eta \dfrac{{VA}}{L}$
Where F is the force required to keep the layers of fluid moving, $\eta $ is the coefficient of viscosity, A is the cross sectional area of the layer of fluid, V is the velocity and L is the distance between the layers.
So, $\eta = \dfrac{{FL}}{{VA}}$
So, higher the velocity or the velocity gradient lower is the coefficient of viscosity $\eta $
Or lower the velocity gradient higher is the $\eta $.
As we go deeper in the flowing river the velocity decreases and so the viscosity increases, so the fluid friction increases.

So the correct option is (B).

Note: The formula which tells the relation between velocity and viscosity is important. Viscosity is the friction experienced when we pour honey from a jar. Honey is highly viscous. When we pour water or juice from a bottle, we can see it flows freely and quickly. It has very low viscosity.