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Bernoulli's Principle

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Bernoulli's Principle - Lesson Summary

Applications of Bernoulli's Theorem

Whenever a flowing fluid speeds up, there is a corresponding decrease in the pressure and/or the potential enegy of the fluid. If the flow is horizontal, the whole of the velocity increase is accounted for by a decrease in pressure. This fact has a large number of practical applications.

Aerofoil lift

An aerofoil(an aircraft wing) is shaped so that air flows faster along the top suface than the lower surface.

Fromr Bernoulli's theorem the pressure of the air below is greater than that of the above this difference in pressure that provides a net upward force, called dynamic lift.

Actually bernoulli's theorem is only one aspect of the lift on a wing. Wings are usually tilted slightly upward so that air striking the bottom surface is directed downward. The change in momentum of the rebounding air molecules results in an additional upward force on the wing.

Atomiser or sprayer

When the bulb in the atomiser is squeezed, air rushes into through the narrow neck of the atomiser. This reduces the pressure in the narrow channel. So that the atmospheric pressure pushes the perfume up the tube the tube leading to the narrow channel. The perfume is then dispensed in to a fine spray of droplets.

Blowing off roof in wind storm

When wind flows with a velocity above the roof of the a house, it causes a lowering of pressure above the roof.

However, pressure below the roof is still atmospheric. If the wind blows fast enough, the internal air pressure (atmosphere) can blow the roof off.

Calculation of the upward force on the roof pushes down with a strom

Let the air above the roof pushes the roof down with force,

F1 = P1A, where P1 is the air pressure above the roof and A is the area of the roof.

The air inside the house pushes up with a force F = PaA, where Pa is the atmospheric pressure of the air inside the house. The net upward force is

Fnet = F – F1 = PaA – P1A = A(Pa – p1)

A small pressure difference over a large area can produce a large upward force.

A suction effect is experienced by a person standing close to the platform at a station when a fast train passes. The fast moving air between the person and the train produces a decrease in pressure and the excess air pressure on the other side pushes the person towards the train.

Smoke go up a chimney

Smoke go up a chimney. It is partly because hot air rises (i.e, density decreases). But Bernoulli's theorem explains it.

The wind blows across the top of a chimney, the pressure is less than inside the house. Hence air and smoke are pushed up the chimney.

On an apparently still night, there is usually enough ambient air flow at the top of a chimney to allow upward flow of smoke.

Flying of a Bird

A bird's wings are curved along the top, when air passes over the wing, the curve divides the air forces on top to travel a greater distance than the air on the bottom. The tendency of airflow, is to correct for the presence of solid objects and to return to its original pattern as quickly as possible.

The rate of flow at the top increases when the air hits the front of the wing, to compensate for the greater distance. It has to travel than the air below the wing. According to Bernoulli's principle, fast moving fluid exerts less pressure than slow-moving fluid; therefore, there is a difference in pressure between the air below and the air above, and this keeps the wing aloft.

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