Why does a thin strip of paper rise when we blow air over it?

What causes a thin strip of paper to rise when air is blown over it?

A. The pressure above the strip is higher than the pressure below the strip.

B. The pressure above the strip is lower than the pressure below the strip.

C. Atmospheric pressure changes with altitude.

D. None of these.

Answer:

The strip of paper rises when air is blown over it because the pressure above the strip becomes lower than the pressure below the strip, based on Bernoulli's principle. The answer to the question is B.

When we blow air over a thin strip of paper, it rises due to a principle in physics known as Bernoulli's principle. This 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. So, blowing across the paper increases the speed of the air flow over the strip. According to Bernoulli's principle, this should lower the air pressure above the strip. The pressure beneath the strip remains relatively higher because the air is not moving as fast. The higher pressure below then pushes the paper upward, causing the strip to rise.

The correct answer to the student's question is: B. The pressure above the strip is lower than the pressure below the strip.

Experimentation with Two Strips of Paper

To illustrate this phenomenon, one can perform a simple experiment with two strips of paper. By holding the small end of one strip up to your lips and blowing across, you will observe the rising effect due to the lower pressure above it. Similarly, blowing between two strips of paper held closely together will cause them to come towards each other, again due to the lower pressure that is created between them with the fast-moving air.