Put a straw in a glass of drinking water. Suck the straw with your mouth. What happens to the air pressure in the straw? Why does the water go up into your mouth?

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Solution

When you put straw in the glass of drinking water and suck on the straw, the pressure of air inside the mouth gets reduced that in turn lowers the pressure of air at the top of the straw. As a result of this the pressure of air inside the straw is less than the outside pressure exerted by atmosphere on water in the glass. The higher atmosphere force or pressure outside pushes the water up the straw into the mouth.

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Question 3

Figure shows a child blowing air with a straw near the opening of another straw which has its other end in a soft drink bottle. It was observed that the level of the soft drink in the straw rises up as soon as air is blown over its open end. Which one of the following best explains the reason for rise in level of the drink?

(a) Blowing of air decreases pressure over the opening of the straw.

(b) The straw of the soft drink bottle collapses when air is blown over its open end

(d) Blowing of air increases the pressure on the surface of soft drink in the bottle.

If you blow air over the top of a fairly large drinking straw you can hear a fundamental frequency due to a standing wave being set up in the straw. What happens to the fundamental frequency if while blowing you cut the straw in half with a pair of scissors?

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