Question

Assertion (A): When a thin flat plate is held parallel to a fluid stream, both pressure drag as well as viscous drag will be negligible.
Reason (R): The total drag on the plate will be entirely due to shear stress and pressure drag will be negligible.

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. (A) is correct but (R) is not correct
• D. (A) is not correct but (R) is correct

Hint:

Parallel plate → only skin friction drag exists, not pressure drag.

Answer 1

The Correct Option is D

Concept: Drag force on a body in fluid flow consists of:
• Pressure drag (form drag)
• Viscous drag (skin friction drag)

Step 1: Analyze given configuration.
Plate is parallel to flow:
• Flow remains attached
• No significant pressure difference across plate

Step 2: Pressure drag.
Since no separation: \[ \text{Pressure drag} \approx 0 \]

Step 3: Viscous drag.
However:
• Fluid sticks to surface (no-slip condition)
• Shear stress develops Thus viscous drag is NOT negligible.

Step 4: Evaluate Assertion (A).
It says both pressure and viscous drag negligible → Incorrect.

Step 5: Evaluate Reason (R).
States drag is due to shear stress only → Correct. Final Answer: \[ \boxed{\text{(A) is not correct but (R) is correct}} \]