Lifting Paper With Moving Air

This simple experiment helps explain one aspect of how airplanes can fly even though they are heavier than air.

No substantial safety risks.

Required

• Thin strip of paper (at least 3 inches by 1 inch or so)

• Flat surface (desk/table)

Optional

• Variable Speed Fan
• Tape

• Two books or blocks of equal height

1. Hold or tape the piece of paper such that the short edge is lined up with the ege of the table.

1. Put your mouth at the same level as the top edge of the table, and blow parallel to the piece of paper, such that air passes over the paper.

You should see the piece of paper lift up off the table.

Optional

*Try using an adjustable speed fan, or try adjust how hard you blow. How does the air/wind speed affect the lift of the paper. Is there an ideal angle at which to blow?

*You can do the following similar experiment:

1. Put a piece of paper resting like a bridge on two books/blocks at the edge of a desk.
2. Blow underneath the piece of paper, not at it, but parallel to it.

1. Blow above the piece of paper, not at it, but parallel to it.

The paper should dip down when you blow beneath it, and either not move, or slightly rise when you blow above it.

Q: What did you observe?

A: The far end of the piece fo paper lifted off the table

Q: What caused this to happen?

A: Air flowing over the paper

Q: Why does moving air lift the paper?

A: Bernoulli’s law, see below

Q: Can you think of any examples where moving air is used to lift or push something?

A: Airplane

Q: How does this help explain how an airplane flies?

A: (See below)

Q: What forces pressed down on the paper?

A: Gravity, air pressure

Q: What forces pressed up on the paper?

A: Pressure from table, and once air got under the paper, air pressure from underneath

Air Exerts Forces

Air is really made of molecules of various gases, primarily nitrogen (78%), oxygen (21%), and other gasses such as argon and carbon dioxide (less than 1% each). At room temperature these molecules are constantly moving around, and bumping into eachother, like a room full of bouncy balls. When these molecules hit anything, they exert a small force – and while it is very small, there are a LOT of molecules in air hitting surfaces all the time. At sea level, air pressure is about 14.7 pounds per square inch, meaning that your piece of paper had a lot of “weight” of air pressing down on it.

Bernoulli’s Law Says That Moving Fluids Exert Less Pressure

Bernoulli’s Law states that a flowing fluid exerts less pressure. When you blow over the paper, the air above it moves away (and more air fills in the space). But quickly moving air is less dense and collides less with the paper underneath it. Because there are fewer collisions pushing the paper down, there is less force pushing it down.

What Pushes The Paper Up?

Like air, yourtable is made of molecules, or atoms, that are constantly jiggling around. They don’t jiggle much compared to air, but they do move some. When you put your hand on a table, the table (gently) pushes up against you as its particles collide with your hand? Think about it – Why can you feel the table? Because it pushes against you, as you push against it. Once the piece of paper begins to lift, air goes underneath it. This air also exerts pressure, but unlike the air above the paper, this air is still, so it exerts more pressure UP than the moving air exerts down.

How Does This Apply To Airplanes?

Airplane wings are designed such that air flows more quickly over the top of them than over the bottom. This means the air underneath the wing, which is moving slower, exerts more pressure UP then than the air above the wing does pushing down. Airplanes have big wings because the force exerted is per square inch, so a bigger wing experiences more lift, as long as the wing is designed to have the air above it flow more quickly than that beneath it.
IT IS IMPORTANT TO NOTE that this is just one source of lift in an airplane. It is believed that a much larger lift comes from the wing reorienting air downward. See references for more information on the rest of airplane flight.

None.

Very good discussion of how airplanes fly, discussing how Bernoulli’s Law helps, but does not explain the whole story. This site offers a fuller explanation of flight.

A less in depth, but easier to understand explanation is here.

Experiment changed to reflect Bobby’s comments. Based on a survey of many resources on airplanes and Bernoulli’s law, it seems that Wikipedia is perhaps not telling the whole story? (See references above)

I’d love to have a specialist in the field check over this experiment. It currently beats most online material, but I’d love to have the extra bit that a specialist could provide.