In fluid mechanics, the frictional force exerted by a moving fluid on a body or by layers of fluid on each other is called what?

Introduction / Context:
Whenever an object moves through a fluid such as air or water, it experiences a resistive force that opposes its motion. This force is an important concept in physics, engineering, and everyday life. The question asks you to recall the specific name given to the frictional force exerted by fluids, which is a standard term in fluid mechanics and aerodynamics.

Given Data / Assumptions:
- The medium is a fluid, which can be a liquid or a gas.- There is relative motion between the object and the fluid or between fluid layers.- We are interested in the resistive friction like force caused by this motion.

Concept / Approach:
In solids, friction acts at the surfaces in contact. In fluids, the corresponding resistive force due to viscosity and flow is called drag. Drag acts opposite to the direction of motion and depends on factors such as shape, speed, fluid density, and viscosity. It is crucial for designing aircraft, cars, ships, and even sports equipment. Other fluid related forces include buoyancy, which acts upward due to displaced fluid, and upthrust, which is another word for buoyant force, but these are not frictional forces along the direction of motion.

Step-by-Step Solution:
1. Identify that the question is asking about a frictional force in a fluid, not in a solid.2. Recall the term drag, which is defined as the resistive force exerted by a fluid on a body moving through it.3. Compare this definition with other fluid related terms such as buoyancy and upthrust, which act vertically upward.4. Recognise that convection is a mode of heat transfer in fluids, not a mechanical frictional force.5. Conclude that drag is the correct specific name for the frictional force exerted by fluids.

Verification / Alternative check:
Think about common phrases like aerodynamic drag, water drag on swimmers, or air resistance on cyclists. In all these cases, the term drag is used to describe the fluid friction that slows motion. Textbooks in mechanics and fluid dynamics consistently define drag in this way, confirming that it is the correct term.

Why Other Options Are Wrong:
- Buoyancy: This is the upward force on a body submerged in a fluid due to displaced fluid, not a friction force.- Upthrust: This is another name for buoyant force and again acts vertically upward, not along the path of motion.- Convection: This describes motion of fluid due to temperature differences and is a heat transfer process, not a resistive force.

Common Pitfalls:
Students often confuse all fluid related forces and assume any fluid force is buoyancy. It is important to distinguish between vertical buoyant forces and horizontal or directional resistive forces. Remember that the word drag is often used in practical engineering contexts, so associating it with resistance in fluids will help avoid confusion.

Final Answer:
The frictional force exerted by fluids on moving bodies or between fluid layers is called drag.