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Sometimes people remove letters from envelopes by cutting a sliver from a narrow end, holding the envelope firmly, and blowing toward it. Explain, using Bernoulli's equation, why this procedure is successful.

Sometimes people remove letters from envelopes by cutting a sliver from a narrow end, holding the envelope firmly, and blowing toward it. Explain, using Bernoulli's equation, why this procedure is successful.

Grade:upto college level

1 Answers

Navjyot Kalra
askIITians Faculty 654 Points
8 years ago
When air is blowing towards the opening of the envelope, the speed of the air inside the envelope is decreased. But the speed on the side of the envelope increases. According to Bernoulli’s equation, the pressure of the flow of fluid increases where the pressure decreases and the converse. So, the pressure inside the envelope increases and the pressure sideways to the envelope decrease. There is a difference in the pressure between the inside and outside of the air flow in the envelope. So, the high pressure air inside the envelope is tending to rush outside to the low pressure region. This makes the envelope to open.

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