Plastic cups in love – experimental demonstration of Bernoulli’s principle
Plastic cups in love
What does a flying airplane and a pitcher’s curve ball have in common. Well, besides the fact that both travel through the air at amazingly fast speeds – both are based on a principle called Bernoulli’s principle. Bernoulli, (pronounced Burr New Lee) was a Swiss mathematician who liked to piddle around with these types of things. Now we get to piddle too…
- Out of the string, make 2-1 foot long pieces.
- Using tape, attach one end of the string to the bottom of one of the cups.
- Take the other end of the string and attach it to a table.
- Repeat steps 2 and 3 with the second cup. When taping this string to the table, make sure that the cups will hang off the table two inches apart and at the same height.
- Blow between the two cups.
What happens? Bernoulli’s principle states that in areas where air moves rapidly, pressure is low. When you blow between the cups you are lowering the air pressure between the cups. Blowing between the cups drops the pressure so the higher air pressure of the surrounding air pushes the cups together. This same principle explains how a airplane can fly (due to the shape of the wing) and how a pitcher can throw a curve ball (due to the spin of the ball).
Bernoulli’s principle, also called Bernoulli’s law or Bernoulli’s theorem, states that energy is conserved in a moving fluid (liquid or gas). If the fluid is moving in a horizontal direction, the pressure decreases as the speed of the fluid increases. If the speed decreases, the pressure increases. For example, water moves faster through a narrow portion of a horizontal pipe than through a wider portion. Bernoulli’s principle predicts that the pressure will be lowest where the speed is greatest. Bernoulli’s principle was named for Daniel Bernoulli (1700-1782), a Swiss mathematician.
Bernoulli’s principle can explain how airplane wings create the upward force called lift and how a baseball pitcher can throw a curve ball. An airplane wing is shaped so the air speed above the wing is greater than the air speed below. This means the air pressure below the wing is greater than the pressure above, and the wing is pushed upward. In throwing a curve ball, a pitcher makes the ball spin fast. As a result, the air speed is greater on one side of the ball than on the other. The resulting difference in air pressure produces a net force toward the lower-pressure side and pushes the ball along a curved path.