A calendar hangs from a nail on a wall with a small gap between it and the wall. If air is blown through the gap between the calendar and the wall, what happens to the calendar?

the calendar moves closer to the wall
the calendar moves away from the wall
the calendar stays in the same position
the calendar may or may not move

Correct answer: the calendar moves closer to the wall

Solution

Blowing air through the gap increases the air speed between the calendar and wall, lowering the pressure there (Bernoulli's principle). The still air on the front face is at higher (atmospheric) pressure, so the net force pushes the calendar toward the wall.

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