Pressure: Common Mistakes (Form 5 SPM Physics)

The mistakes, and the fix

1. P = F / A, Area must be in m² (convert cm²: 1 cm² = 10⁻⁴ m²). Recall that Pa = N m⁻².
2. P = hρg, h is the vertical depth, not the distance along a slope. This is only the liquid pressure, add atmospheric pressure for the total pressure.
3. F₁ / A₁ = F₂ / A₂, The force is multiplied in the ratio of the areas. Keep both areas in the same unit.
4. Fb = ρVg, Use the density of the fluid, not the object. V is the volume of fluid displaced (the submerged volume).
5. In the How the depth of a liquid affects the pressure practical, skipping this: Measure the depth to the centre of the membrane.
6. In the How the depth of a liquid affects the pressure practical, skipping this: Read both manometer levels at eye level to avoid parallax.
7. In the Measuring gas pressure with a liquid manometer practical, skipping this: Read both liquid levels at eye level to avoid parallax error.
8. In the Measuring gas pressure with a liquid manometer practical, skipping this: Make sure the tubing has no leaks.
9. Mixing up Hydraulic system and Archimedes' principle. Hydraulic system: A system that uses Pascal's principle to multiply force by transmitting pressure through an enclosed liquid, as in a hydraulic jack. Archimedes' principle: An object wholly or partly immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces.
10. Mixing up Buoyant force and Principle of flotation. Buoyant force: The upward force exerted by a fluid on an object immersed in it. Principle of flotation: A floating object displaces a weight of fluid equal to its own weight.
11. Mixing up Bernoulli's principle and Manometer. Bernoulli's principle: For a flowing fluid, where the speed is higher the pressure is lower, and where the speed is lower the pressure is higher. Manometer: A U-shaped tube containing liquid used to measure the pressure of a gas by comparing liquid levels in its two arms.
12. Mixing up Pressure and Pressure in liquids. Pressure: The force acting normally (perpendicularly) on unit area of a surface (pressure = force ÷ area); measured in pascals (Pa). Pressure in liquids: The pressure at a point in a liquid, which increases with depth and density (pressure = height × density × gravitational field strength) and acts equally in all directions.
13. Mixing up Atmospheric pressure and Gas pressure. Atmospheric pressure: The pressure exerted by the weight of the column of air in the atmosphere on the Earth's surface and on objects in it. Gas pressure: The pressure produced by the collisions of gas particles with the walls of their container.
14. Dropping the unit on the final line, an answer without its SI unit is incomplete.
15. Rounding early: keep full calculator values and round only the final answer to sensible significant figures.
16. Answering "describe" with a definition, describe means say what happens; define means say what it is.

How a 1-to-1 teacher uses this list

In lessons the teacher marks a student's own work against this list, then re-teaches only the items that actually appeared, which is why 1-to-1 fixes habits that group classes miss.

More Pressure resources

• Chapter overview: Pressure
• Revision Notes
• Practice Questions
• Paper 2 Answering Guide
• Paper 3 Guide
• Key Terms
• Calculation practice sets

Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)