Bernoulli's principle states that where a fluid moves faster, its pressure is lower. An aeroplane wing is shaped so that air flowing over the curved upper surface travels faster than the air passing beneath the flatter lower surface.
Because the air on top moves faster, its pressure is lower than the pressure below the wing. This pressure difference produces a net upward force, lift, that can be large enough to hold the whole aircraft up. Tilting the wing slightly also deflects air downward, adding to the lift.
The same effect explains why a shower curtain drifts inward when the water runs, and why two boats moving side by side can be drawn together. A pilot increases speed and adjusts the wing angle to raise lift for take-off.
In SPM you should connect faster flow with lower pressure and use it to explain lift and similar everyday effects.
Common misconceptions
- Lift comes only from air hitting the bottom of the wing -> A major part comes from lower pressure above the wing due to faster airflow.
- Faster-moving air has higher pressure -> Bernoulli's principle says faster flow means lower pressure.
- A wing must be tilted a lot to fly -> Even a small tilt works with the shaped upper surface; large tilts can stall the wing.
The physics behind it
Bernoulli's principle states that where a fluid flows faster, its pressure is lower. A wing (aerofoil) is curved on top and flatter below, so air over the top travels faster and its pressure falls below the pressure acting on the underside.
The pressure difference across the wing, multiplied by the wing area, gives the upward lift force: F = ΔP × A, where pressure is in pascals (Pa = N m⁻²) and area in m².
For a wing of area A = 30 m² with a pressure difference ΔP = 2000 Pa, the lift is F = ΔP × A = 2000 Pa × 30 m² = 60000 N. Tilting the wing (the angle of attack) also deflects air downward, and by Newton's third law the air pushes the wing up, adding to the lift. Both descriptions matter: reduced pressure above and downward deflection of air below.
Lift grows with speed, so an aircraft accelerates along the runway until lift exceeds its weight.
See it in daily life
Hold a strip of paper just below your lower lip and blow across the top. Instead of drooping, the free end rises.
The fast air you blow over the upper surface has lower pressure than the still air beneath, so the higher pressure below lifts the paper, exactly the aerofoil idea in miniature.
The same effect tugs a shower curtain inward when the water runs: the moving spray lowers the pressure inside, and the higher pressure of the still bathroom air pushes the curtain in. Two boats moving side by side in a river can be drawn together, because the water squeezed between them speeds up and its pressure drops.
A perfume atomiser and the air-hole of a Bunsen burner work the same way: fast-moving air or gas makes a low-pressure region that draws liquid or air in. None of these needs an engine pushing sideways; the motion of the fluid alone changes the pressure and produces a real force.
How this comes up in SPM
This sits in the Form 5 Pressure chapter, under Bernoulli's principle and its applications. Paper 2 typically asks you to State Bernoulli's principle, then Explain how the shape of an aerofoil produces lift by relating the speed of air to the pressure above and below the wing.
Diagram questions may show streamlines closer together above the wing and ask you to Compare the pressures, or to Mark the direction of the lift force.
You may be asked to Relate lift to aircraft take-off, or to Explain a second application such as a Bunsen burner, an insecticide sprayer or a curtain drawn inward. Neighbouring standards include pressure in liquids, atmospheric pressure, gas pressure and Pascal's principle, so command words like Define, State, Explain and Relate recur across the chapter.
Answers that only mention faster air, without naming the lower pressure and the upward pressure difference it causes, usually lose marks, so complete the chain from speed to pressure to force.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)