A circular disc 3 m in diameter is held normal to a 26·4 m/s wind of density 1·2 kg/m³. If the coefficient of drag of disc is 1·1 the force required to hold it at rest will be nearly
- A1·25 kN
- B2·5 kN
- C3·25 kNCorrect
- D4·2 kN
Solution
Concept
Drag force
Formula
FD = CD · ½ ρ A V²
Step-by-step solution
1. A = π × 3² / 4 = 7.07 m². 2. F = 1.1 × 0.5 × 1.2 × 7.07 × 26.4² = 3.25 × 10³ N = 3.25 kN.
Explanation
The drag on a bluff body is the drag coefficient times the dynamic pressure and the projected area.
Why the other options are wrong
The other values do not match 0.66 × 7.07 × 696.96 = 3252 N.
Shortcut
F = Cd × ½ρV² × A.
Common mistake
Using the diameter instead of the area.
Correct answer: C) 3·25 kN