In the falling sphere method,
- AStoke’s law is used to measure the viscosity of a fluidCorrect
- BNewton’s law of viscosity is used to measure the viscosity of a fluid
- CHagen-Poiseuille equation is used to measure the viscosity of a fluid
- DNone of the above
Solution
Concept
Falling sphere viscometer works on Stokes' law
Formula
μ = 2 r² (ρs − ρf) g / (9 V)
Step-by-step solution
1. A small sphere falls through the liquid at its terminal velocity. 2. At terminal velocity weight − buoyancy = drag from Stokes' law. 3. Solve for the viscosity μ.
Explanation
Stokes' law gives the drag force on a small sphere moving slowly in a viscous fluid. Measuring the terminal velocity of the sphere gives the viscosity.
Shortcut
Falling sphere = Stokes. Capillary tube = Hagen-Poiseuille. Rotating cylinder = Newton.
Common mistake
Choosing Newton's law because it is the definition of viscosity.
Correct answer: A) Stoke’s law is used to measure the viscosity of a fluid