Question

Explain why to keep a piece of paper horizontal, you should blow over, not under, it?

Answer 1

To keep a piece of paper horizontal, one should blow over it. This increases the velocity of air above the paper. As per Bernoulli's principle, atmospheric pressure reduces above the paper and the paper remains horizontal.

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Answer 2

Answer: To compensate for weight of paper, a reduced pressure region is created above the paper by blowing over it.

Explanation:

According to bernoullie's theorem of fluids:

P + 1/2(ρv^{2} ) = constant

So when velocity of air above paper is increased, the pressure over it decreases. So the higher pressure from under the paper provides an upward force which balances the downward force due to gravity on the paper