Free Fluid Drag MCQs with Answers

1. According to Stokes's law, the drag force on a small sphere moving slowly through a viscous fluid is proportional to

• A. the square of the speed
• B. the speed
• C. the cube of the radius
• D. the inverse of the radius

Explanation: Stokes's law gives the drag as 6 pi eta r v, so it rises in direct proportion to both the radius and the speed, provided the flow stays laminar. At higher speeds the flow becomes turbulent and drag grows roughly with the square of the speed instead. The law is what allows viscosity to be measured by timing a falling ball.

2. The SI unit of coefficient of viscosity is

• A. N s per m squared
• B. N per m squared
• C. N m
• D. m squared per s

Explanation: Viscosity is defined through the relation between shear stress and velocity gradient, giving units of newton second per square metre, equivalently the pascal second. The pascal, newton per square metre, is pressure rather than viscosity. In the older CGS system the unit was the poise, and one pascal second equals ten poise.

3. As the temperature rises, the viscosity of a liquid

• A. increases
• B. decreases
• C. stays the same
• D. becomes zero

Explanation: Heating gives the molecules enough energy to slip past one another more easily, weakening the intermolecular attractions that resist flow, which is why warm honey pours readily. Gases behave in the opposite way: their viscosity rises with temperature because it comes from molecular collisions rather than cohesion. Confusing the two cases is the usual mistake.