Question

Let us assume that the gale is blowing over a house. What will be the force due to gale on the roof?
A. towards downward direction
B. towards upward direction
C. zero
D. towards horizontal direction

Answer 1

Hint: This question can be answered based on a principle called Bernoulli's principle. This principle says that fast-moving fluids or air like strong winds, will have a lower pressure than slow-moving air. In a gale the fast flow of air above the shape of the roof creates a low-pressure area. This will help you in answering this question.

Complete answer:
At the time of a storm, the roof will be blown off making no damages to the other parts of the house. Based on Bernoulli's principle, the storm blowing over the roof will produce a low-pressure ${{P}_{1}}$. The pressure under the roof ${{P}_{2}}$ will be greater. Hence this pressure difference $\left( {{P}_{2}}-{{P}_{1}} \right)$ will generate an upthrust and the roof will be blown off. In this question a gale is mentioned. The gale would be blowing means that the area under the gale will be having a lower pressure than that over the house. Hence the force will be experienced in a direction from higher pressure to lower pressure. That is in the upward direction.

Therefore the correct answer is option B.

Note:
In ancient times, the roofs of the huts or the houses were made with a slope. The main scientific reason for this will be as per Bernoulli's principle. This is to safeguard except roofs at the time of storms or cyclones. The wings of an airplane also known as aerofoil are made that its upper surface is more curved than the lower surface and the front edge is broader than the real edge. This is also based on this principle.