In a moving fluid, pressure is lower where the speed is higher. This is Bernoulli’s principle, and SPM questions ask you to use it to explain what happens, not to calculate it.
This lesson is part of SPM Physics pressure. It follows applying Archimedes’ principle.
What four steps make a full explanation?
Each step in the chain earns a mark. Use the same four steps every time.
- State where the fluid moves faster.
- State that the pressure is lower there (Bernoulli’s principle).
- State that the pressure on the other side is higher, so there is a pressure difference.
- State the net force and its direction, and the effect on the object.
Worked example: two hanging paper strips
An original activity hangs two paper strips a few centimetres apart. You blow air between them and they move together.
- The air between the strips moves faster than the still air on the outside.
- By Bernoulli’s principle, the pressure between the strips is lower.
- The pressure outside is atmospheric, which is higher, so a pressure difference exists.
- The net force acts inward on each strip, so they move toward each other.
How does the frame work on an aerofoil?
An aircraft wing is shaped so that air moves faster over the curved top surface than under the wing. The pressure above the wing is lower than the pressure below it.
The pressure difference gives a net upward force, called lift. When the aircraft moves fast enough, lift is greater than weight and it rises.
How does the frame work on a spray bottle?
Squeezing the bulb sends air rushing across the top of a tube that dips into the liquid. The fast air lowers the pressure at the tube’s top end.
The atmospheric pressure on the liquid surface is higher, so the liquid is pushed up the tube. It reaches the airstream and breaks into droplets.
The mistake that costs marks
The slip is to write that the fast air pulls the paper, or that the wind pushes the paper. Neither names a pressure difference.
| Answer | Marks earned |
|---|---|
| Fast air pulls the strips together | Low: names a force, omits the pressure difference |
| Fast air between the strips gives lower pressure; higher outside pressure pushes them inward | Full: speed, pressure, difference, force |
The marker looks for the words “lower pressure” and “higher pressure” and the direction of the resulting force. Write both sides.
What about pipes of different widths?
A fluid flowing steadily through a pipe moves faster in the narrow section, because the same volume passes every second. Bernoulli’s principle then says the pressure is lower in the narrow section.
This is why a Bunsen burner draws air in through its collar: fast gas lowers the pressure there, and the higher outside air pressure pushes air in.
Check yourself
A lorry passes close to a small car on a motorway, and the car is pulled slightly toward the lorry. Explain why, using the four steps.
Answer
- The air between the lorry and the car moves faster than the air on the car’s far side.
- By Bernoulli’s principle, the pressure between them is lower.
- The pressure on the car’s far side is higher, so there is a pressure difference.
- The net force acts toward the lorry, so the car is pushed toward it.
What to study next
Test the whole chapter in the pressure practice set. Return to applying Pascal’s principle if the closed-liquid ideas need another look.
To practise writing structured Physics answers with a teacher, see online one-to-one Physics tuition.