Things need :
v A piece of paper
- Hold a piece of paper by the corners of edge.
- Bring it up to your mouth. The paper will droop towards the floor.
- Blow gently across the top surface of the paper. Does the paper still droop.
- Take a deep breath and blow harder.
- The paper rise. Before you blow, the air on both the sides of the paper is still.
§ The pressure is same on both the sides. When you blow over the upper surface, the pressure on the side is reduced. But the pressure underneath is the same as before. The higher pressure underneath forces the paper up. Blowing harder reduces the pressure on the upper surface even more. This makes the paper rise even higher.