Wind blows at 30 m/s over a flat roof of area 100 m² while the air inside is still. What is the approximate upward force on the roof (density of air = 1.2 kg/m³)?
Correct answer D. 5.4 × 10⁴ N
Explanation
Pressure difference = ½ρv² = ½ × 1.2 × 30² = 540 Pa. Force = ΔP × A = 540 × 100 = 5.4 × 10⁴ N. Forgetting the ½ doubles the answer.
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