Question

Bernoulli’s Principle

This activity will allow you to see Bernoulli’s Principle in action. Some of the effects of fast moving air can be rather surprising.

Equipment

• Cup
• Straw
• Index cards

• Blow Here

  Water

Figure 1: The straws for Procedure 1

Procedure 1

1. Fill a small cup with water.
2. Cut the plastic straw in half. Hold one piece vertically in the water, and the other in your hand.
3. Position the straws so that they are perpendicular to each other, and arranged as shown In Figure 1.
4. Blow briskly into the horizontal straw.

Side note: listen to the sound the air blowing over the straw makes. This will become important in the chapter about sound and music.

Procedure 2

1. Fold one index card in the middle so that it makes a little tent. Fold the other on each end so it is like a little table. Set the cards on a table or desk.
2. Try to blow each card off the table by blowing underneath each one in turn (don’t blow on the side of the tent or table). If needed, use a straw from Procedure 1 to help direct the air below the cards. Note what happens as the air passes underneath the cards.

Questions

1. What happened to the water level in the vertical straw when you blew into the horizontal straw?

2. Explain why this happens according to Bernoulli’s Principle. Describe what the fast moving air does to the pressure above the water in the straw.

3. What happened when you tried to blow under the cards? Were you able to blow them off the table?

4. Again, in terms of the fast moving air and pressure, explain why this happens.

Answer 1

Bernoulli's Principle: Bernoulli's Principle states that as the speed of a moving fluid (liquid or gas) increases, the pressure within the fluid decreases.

Water level in vertical straw: water level in the vertical straw rises up as we blew into the horizontal straw, because, as we understand from Bernoulli's principle, when we blew into the horizontal straw, it increased the velocity of air above the vertical straw which in turn decreased the air pressure with respect to the pressure at the bottom of vertical straw. This pressure difference pulled the water up the vertical straw.

Blowing the cards: When I tried to blow under the cards, it seemed that something is pulling it down and I was not able to blow it off the table. The reason is same as previous, because we are blowing underneath the card the velocity of air underneath the card increased with respect to the air outside the card, which created a low pressure underneath the card with respect to the pressure above the card and this pressure difference pull the cards towards the table and hence I could not blow than off the table.