Not given in the exam, recall it.
What it is for
Pressure is used in Pressure (SPM Physics Form 5). Keep the SI units in every line of working. Pressure
Symbols and SI units
| Symbol | Meaning | SI unit |
|---|---|---|
| P | pressure | Pa |
| F | force | N |
| A | area | m² |
Rearrangements
F = PAA = F / P
Worked example
Common trap
Understanding pressure
P is the pressure in pascals (Pa), F is the force acting perpendicular to a surface in newtons (N), and A is the area over which the force acts in square metres (m²). Pressure measures how concentrated a force is: the same force on a smaller area produces a greater pressure, which is why a sharp knife or a nail cuts more easily.
One pascal equals one newton per square metre (Pa = N m⁻²).
Because the three quantities are linked by P = F / A, you can rearrange to find the force with F = PA, or the area with A = F / P. Always convert the area to square metres before substituting, since a force in newtons divided by an area in square metres gives pressure directly in pascals.
A worked example, step by step
A brick rests on a table and presses down with a force F = 30 N through a face of area A = 150 cm². Find the pressure on the table.
- Convert the area to square metres: A = 150 cm² = 150 × 10⁻⁴ m² = 0.015 m².
- Write the formula: P = F / A.
- Substitute with units: P = 30 N / 0.015 m² = 2000 Pa.
The answer is P = 2000 Pa, or 2.0 × 10³ Pa to two significant figures. The unit check confirms it: N ÷ m² = N m⁻² = Pa.
If the brick were stood on a smaller face, the same 30 N would give a larger pressure.
Common mistakes and how this is tested
The biggest error is leaving the area in cm²; remember 1 cm² = 10⁻⁴ m², not 10⁻² m². Another is using a force that is not perpendicular to the surface, only the component pressing at right angles counts.
- Confusing pressure (Pa) with force (N).
- Forgetting that Pa = N m⁻².
- Dividing area by force instead of force by area.
Questions ask you to calculate the pressure a shoe or blade exerts, explain why a camel's wide feet reduce pressure on sand, or define pressure as force per unit area. For definitions, state that pressure is the perpendicular force acting per unit area.
Show the area conversion line clearly to secure the method mark.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)