Pressure tells you how concentrated a force is: the same force gives a large pressure on a small area and a small pressure on a large area. In fluids (liquids and gases), pressure also depends on depth. This topic joins force, area, density and depth in a few short equations.
Exact wording and required items change between syllabus years, so check the current Cambridge IGCSE Physics 0625 page for your exam year. The Physics learning guide shows where this topic sits in the whole subject.
What do I need to know first?
You need the idea of a force in newtons and the link between mass and weight, which is covered in mass, weight and density. You also need to convert cm² to m² and rearrange a three-term equation. No other Physics is needed.
One worked example that uses the main ideas
A rectangular water tank has a vertical water depth of 1.8 m. A hatch of area 0.050 m² is fixed in the floor. Water density is 1000 kg/m³ and g = 10 N/kg. (Invented example data.)
Choose the relationship: the pressure comes from the liquid, so use p = ρ × g × h.
Pressure at the floor: p = 1000 × 10 × 1.8 = 18 000 Pa.
Force on the hatch: p = F ÷ A, so F = p × A = 18 000 × 0.050 = 900 N.
Check the units: kg/m³ × N/kg × m gives N/m², which is Pa. Then Pa × m² gives N.
Notice that the tank’s width and length never appeared. Only depth, density and g matter for liquid pressure.
In what order should I study the lessons?
- Relate pressure to force and area. Every other lesson uses p = F ÷ A, so start here.
- Calculate liquid pressure under stated conditions. This adds p = ρgh and the idea of vertical depth.
- Explain a pressure difference with depth. It turns the equation into a written explanation, which exam questions ask for.
- Interpret a manometer-style diagram where applicable. It uses liquid pressure to compare two pressures, so it follows the liquid lessons.
- Avoid using mass where a force is required. A short correction lesson that protects every earlier calculation.
Then use the pressure practice set to test all five together.
What traps catch students?
- Mass in place of weight. p = F ÷ A needs newtons, not kilograms.
- Area in cm² left unconverted. 1 m² is 10 000 cm², so 600 cm² is 0.06 m².
- Slant length used as depth. Only the vertical distance below the surface counts.
- Container width treated as important. It does not change liquid pressure.
- Adding or subtracting atmospheric pressure the wrong way in a manometer question.
How do I use the practice set well?
Attempt it after the lessons, with a calculator and a clean page. Write the equation, the substitution and the unit for every answer, then open the worked solution. The set ends with a table that sends each type of error back to the right lesson.
Choosing the right pressure equation and keeping units consistent is a habit that one-to-one teaching can build quickly. Our teachers can look at that in online one-to-one Physics tuition.