When air is blown hard underneath a folded piece of paper resting on a table, the paper is not lifted; instead, it is pressed firmly down against the table surface. This surprising phenomenon occurs because moving fluids create changes in pressure. Whenever a fluid flows at high velocity, a region of low pressure is created. This relationship between fluid velocity and pressure is governed by Bernoulli's principle, which is essential in explaining how lift force is generated on an aeroplane wing, how a Venturi tube functions, and how various everyday devices operate.
Illustrate the inverse relationship between fluid speed and pressure, and show how aerofoil geometry generates lift force.
Bernoulli's principle states that when the velocity of a fluid increases, the pressure in the fluid decreases, and vice versa.
When fluid moves at high velocity, it creates a region of low pressure relative to the surrounding region of high pressure. The resulting pressure difference produces a force directed from the region of higher pressure towards the region of lower pressure.
Blowing across the top of an A4 paper makes air move faster above the paper, producing low pressure above it. The higher atmospheric pressure beneath the paper pushes it upwards. Blowing air below a folded piece of paper presses it flat against the table. Blowing between two suspended inflated balloons pushes them closer together.
A Venturi tube consists of a tube with varying cross-sectional areas and vertical manometric tubes attached along it to indicate fluid pressure at different points.
The pressure at X is higher than the pressure at Z because water flows from X to Z and loses energy along the way. The water column in tube A is therefore taller than the column in tube C, even though both sections have the same cross-sectional area.
An aerofoil has a curved upper surface and a flatter lower surface. Air travelling over the curved top surface moves at a higher velocity than air flowing under the bottom surface. The higher air velocity above creates a region of lower pressure, while the lower air velocity below creates a region of higher pressure. The pressure difference produces an upward resultant force known as lift force.
The angle of attack is the angle between the chord line of the aerofoil and the oncoming airflow. When an aerofoil is tilted at an angle of attack, it deflects oncoming air downwards. According to Newton's Third Law of Motion, the deflected air exerts an equal and opposite upward reaction force on the wing, further contributing to the total lift force.
High-velocity gas flows through a narrow nozzle, creating a region of low pressure inside the burner tube. Higher external atmospheric pressure pushes surrounding air into the air hole to mix with the gas, enabling complete combustion.
The spoiler or diffuser is shaped like an inverted aerofoil. Air travels faster underneath the spoiler than above it, creating a lower pressure below and higher pressure above. This produces a downward resultant force called downforce that presses the car tyres against the track, improving traction and stability at high speeds.
When a football is kicked with a spin, air moves faster on one side and slower on the opposite side. The velocity difference creates a pressure difference across the ball. A resultant force pushes the ball sideways toward the low-pressure side, curving its flight path.
When air is blown downwards into an inverted filter funnel containing a ping pong ball, air flows with high velocity through the narrow space above the ball. This creates a region of high air speed and low pressure above the ball. The region below the ball remains at higher atmospheric pressure. The difference in pressure produces a resultant upward force that keeps the ball supported against gravity inside the funnel.
Answer in your mind, then press "Check Answer".
State Bernoulli's principle.
In a Venturi tube with water flowing steadily from tube A towards tube C, why is the height of the water column in the narrow middle tube B the lowest?
Name the four forces acting on an aeroplane in flight.
Blowing air over a sheet of paper lifts it because the breath physically "pulls" or hooks onto the paper.
Blowing increases air speed over the top surface, creating a low-pressure region above. It is the unreduced, higher atmospheric pressure below the paper that pushes it upward.
Spoilers on racing cars are designed to lift the car and make it lighter to run faster.
Spoilers act as inverted aerofoils to produce downward force (downforce), increasing tyre grip on the track to prevent skidding at high speeds.
Objective: Test your understanding of Bernoulli's principle, Venturi tube behaviour, lift force and its applications.
1 According to Bernoulli's principle, what happens to the pressure within a fluid when its velocity increases?
2 Why does an inverted filter funnel hold a ping pong ball in place when air is blown downwards through its narrow stem?
3 In a continuous fluid flow through a pipe, the smaller the diameter of the pipe, the the velocity of the fluid.
4 An aeroplane wing uses an angle of attack to deflect airflow downwards, producing an upward reaction force in accordance with Newton's Law of Motion.
Match each component or application to its correct description based on Bernoulli's principle. If using a phone, tap the answer first, then tap the matching box.
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