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Chapter 6: Problem 75

An old magic trick uses an empty thread spool and a playing card. The playing card is placed against the bottom of the spool. Contrary to intuition, when one blows downward through the central hole in the spool, the card is not blown away. Instead it is "sucked" up against the spool. Explain.

Short Answer

Expert verified
Using Bernoulli's principle, it can be explained that when one blows downward through the central hole in the spool, the air speed in the hole increases, leading to a decrease in pressure inside. The pressure outside remains unchanged and hence is higher compared to the inside. This pressure difference creates a force that 'sucks' the card up against the spool.

Step by step solution

01

Understand Bernoulli's principle

Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy. Therefore, when one blows downward through the central hole, the air in the chamber (the central hole of the spool) increases in speed.
02

Apply Bernoulli's principle

As a result of the increased speed of air inside the chamber, there is a decrease in pressure according to Bernoulli's principle. This creates a relatively lower pressure region inside the chamber.
03

Understand the effect of pressure difference

The pressure outside the spool remains unchanged and hence is relatively higher than the pressure inside. The card is subjected to the difference in pressure between these two regions.
04

Evaluate the resulting force

The difference in pressure between the two sides of the card creates a force that pushes the card towards the lower pressure region, effectively 'sucking' it up against the spool.

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Most popular questions from this chapter

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