Why are roofs blown off during a cyclone?
Answers
Answer:
The high wind blowing over the roof creates a low pressure in accordance with Bernoulli's principle. The pressure below the roof is equal to the atmospheric pressure which is now larger than pressure above the roof. ... Once the roof is lifted up, it is blown off with the wind along its direction.
Answer:
Pressure is force per unit area, yes, but it also represents the difference in kinetic energy density across a surface - only the energy of random motion of particles, though not large-scale coherent motion like wind. Accordingly, the faster a fluid moves, the more of its kinetic energy goes into large-scale motion, and the less is left for random motion of individual particles, so all other things being equal, faster moving fluids have lower pressure.
You might then wonder why all other things should be equal; in particular, why would air on both sides of a roof have roughly the same overall kinetic energy density? That's because air tends to come to equilibrium over long time scales: the pressure and temperature inside and outside the house, and thus the kinetic energy density, will tend to be similar. When a strong wind kicks up, the pressure outside drops, but there isn't time for air to flow in and out of the house to cause the corresponding drop in the interior pressure. That's why you get a pressure gradient across the roof.
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