Question

There is a film of fluid between the flat plates. On which of the following parameter the force required to move the plates does not depend?
1. velocity of plates
2. area of plate
3. distance between the plates
4. surface tension of fluid

Answer 1

Correct Answer - Option 4 : surface tension of fluid

CONCEPT:

• Viscosity: The state of being sticky, thick, and semi-fluid inconsistency, due to internal friction of fluids is called viscosity.
• Newton's Formula of the coefficient of viscosity:
• According to this formula, force is proportional to contact area A and velocity gradient \({Δ v \over Δ z}\)

\(F \propto A{Δ v \over Δ z}\)

\(F =\pm η A{Δ v \over Δ z}\)

where η is the coefficient of viscosity, A is the contact area, and \({Δ v \over Δ z}\)is velocity gradient.

EXPLANATION:

The force required to drag the plate:

\(F =\pm η A{Δ v \over Δ z}\)

• This force depends on 

1. coefficient of viscosity μ 
2. area of plate A
3. the velocity of plate Δv
4. the distance between the plates Δz

• From the given options, it is clear that the force required to drag the plate does not depend on the surface tension of the fluid.
• So the correct answer is option 4.