# Fluid Question

1. Apr 10, 2006

### sid_galt

I got this question on my physics test

"What will happen if you blow over a piece of paper?"

I answered that with enough blowing, the wind created would make the paper fly off. Apparently according to my teacher, the correct answer was that from Bernouilli's theorem, the air velocity above the paper will increase, the pressure will decrease and the paper will rise up.

But how is that possible? Bernoulli's theorem applies only to a moving fluid whose speed is increased by decreasing the cross sectional area. In blowing, we are creating a high pressure inside the mouth and then releasing the air. The static pressure of the air remains the same all the time. Besides, a paper is held down due to gravity and air pressure. To lift a paper by creating low pressure on top of it would require quite a lot of low pressure plus quite a bit of normal pressure air below the paper.

2. Apr 10, 2006

### siddharth

Tried blowing over a piece of paper?

3. Apr 10, 2006

### sid_galt

yeah. no effect except that the paper gets pushed across the table. But no vertical motion.

4. Apr 10, 2006

### Galileo

That's a crummy way to demonstrate Bernoulli, since you can blow away the paper. Try it like this:
Hold a piece (~A4) of paper in each hand vertically next to each other, about 5 cm apart. Hold them at the top with the long side down. Now blow a strong, steady column of air (taut your lips) between the sheets. What will happen according to Bernoulli?