Bernoulli’s principle Sailing Cutting through to the essence Commentary

Daniel Bernoulli physics

Bernoulli’s principle

• Liquids and gases increasing in motion
• have the energy of their increase
• subtracted from their pressure
• controlling the flow of fuel into a carburetor
• and lifting the wing of an airplane
• so that the sum of all forms of energy remains constant.

Sailing

• Some might think a wind,
• filling a sail,
• pushes the boat, but this is not so.
• The sail is only a flexible airfoil
• whose curve forces the air passing on one side
• to flow more quickly than the air passing on the other,
• so that the increase in speed
• is compensated by a decrease in pressure
• that pulls the boat through the sea, even against the wind.

Cutting through to the essence

• Bernoulli’s principle, stated for an ideal fluid—
• with no viscosity, no drag, and no compressibility,
• is true enough to help with understanding,
• true enough to lift an airplane
• even though all kinds of odd things happen
• as air swirls past.