| English | Bernoulli's principle |
|---|---|
| Bahasa Melayu | Prinsip Bernoulli |
| 中文 | 伯努利原理 |
Definition
For a flowing fluid, where the speed is higher the pressure is lower, and where the speed is lower the pressure is higher.
What you need to know
Bernoulli's principle states that within a moving fluid, regions where the fluid speed is higher have lower pressure, while regions where the fluid speed is lower have higher pressure. This inverse relationship between fluid speed and pressure applies to both liquids and gases in motion.
This principle is mostly examined through describing and explaining everyday applications rather than through calculation. On an aerofoil, or aeroplane wing, the shape causes air to travel faster over the curved upper surface than under the flatter lower surface.
Faster-moving air above the wing has lower pressure than the slower-moving air below it, and this pressure difference produces an upward force called lift.
Other common applications include the Bunsen burner, where fast-moving gas leaving a jet creates a region of lower pressure that draws in air through side holes to improve combustion, and the insecticide spray, where air blown rapidly across the top of a tube lowers the pressure there, drawing liquid up the tube to be sprayed out. A spinning ball curving in flight is also explained using Bernoulli's principle.
Worked example
Bernoulli's principle is typically applied descriptively rather than numerically in the 4531 syllabus, so this worked example uses a labelled explanation of an aeroplane wing instead of a calculation.
Consider air flowing past an aerofoil-shaped wing. The upper surface of the wing is more curved than the lower surface, so air travelling over the top must move a greater distance in the same time as air travelling underneath, meaning the air above the wing moves at a higher speed.
According to Bernoulli's principle, this higher speed above the wing corresponds to lower pressure above the wing, compared with the higher pressure of the slower-moving air beneath it. Because the pressure below the wing is greater than the pressure above it, a net upward force, called lift, acts on the wing.
When this lift force becomes equal to or greater than the weight of the aeroplane, the aeroplane can rise and stay airborne.
How it is examined
In Paper 1, objective questions often ask candidates to identify which situation shows lower pressure due to higher fluid speed, or to label the correct region of high or low pressure on a diagram of an aerofoil. In Paper 2, structured and essay questions typically ask candidates to state Bernoulli's principle, explain how lift is produced on an aeroplane wing, and describe how a Bunsen burner or insecticide spray uses the same principle.
In Paper 3, practical or application-based questions may ask candidates to describe a simple demonstration, such as blowing air between two suspended sheets of paper, and explain the resulting observation using Bernoulli's principle.
A common mistake is reversing the relationship and stating that higher speed causes higher pressure, or forgetting to link the pressure difference clearly to the resulting force in a full explanation.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)