Which term describes the resistance when a solid surface moves through a fluid?

• A. Friction
• B. Fluid Friction
• C. Drag
• D. Resistance

Answer: (B)

The concept being tested is what we call the resistance a solid experiences when moving through a fluid. That resistive force is drag. Drag represents the overall pushback from the fluid, and it comes from two main sources: viscous drag, which is due to the fluid’s thickness or viscosity rubbing against the surface, and pressure drag, which comes from how the fluid is pushed around the object's shape creating pressure differences. As the object speeds up, drag increases, typically with the square of the speed in many situations.

Friction is a general term for contact resistance between solid surfaces, so it isn’t the standard way to describe resistance in a fluid. The more precise term for this situation is drag, sometimes described in parts as viscous drag and form (pressure) drag.