SEC G3 Combined Science Physics component K326/K327
CP3: Force and Pressure
Keep mass, weight, density, force, contact area, and pressure distinct before choosing a relationship.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Force, mass, weight, density, and pressure answer different questions, so first identify whether the evidence concerns interaction, matter, volume, or contact area.
Forces and gravitational fields
Contact forces act when bodies or materials touch. Examples include friction, air resistance or drag, tension, and a normal reaction. Gravitational, electrostatic, and magnetic forces can act across a separation.
Mass measures the amount of matter. A gravitational field is a region where a mass experiences gravitational force, and field strength is the force exerted per unit mass at a point.
Weight and density
Weight is the gravitational force on a mass, so it changes when field strength changes. Mass does not change merely because the object moves to a different gravitational field.
Density compares the mass of a sample with the volume it occupies. Convert mass and volume to compatible units before comparing materials.
Pressure on a surface
Pressure is the normal force distributed across the contact area. The same force gives a larger pressure when it acts on a smaller area.
Use the area actually touching the surface. A diagram may show other dimensions that do not belong in the pressure calculation.
Formulae and relationships
| Relationship | Use |
|---|---|
| Relate mass, field strength, and weight. | |
| Find density. | |
| Find pressure on a surface. |
Worked examples
Example 1: A crate rests on . Take . Find the pressure.
- Find weight: .
- Use the contact area, .
- Calculate .
Answer: .
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Separate the object, external forces, mass, volume, and contact area before selecting a relationship.
- Keep gravitational field strength, weight, density, and force per area as distinct quantities.
- Keep the force-and-pressure model distinct from the resultant-force model used in CP4 Dynamics.
Official outcome coverage
K326 CP3: 9 mapped outcomes, references CP3(a), CP3(b), CP3(c), CP3(d), CP3(e), CP3(f), CP3(g), CP3(h), CP3(i). Check the official K326 syllabus.
K327 CP3: 9 mapped outcomes, references CP3(a), CP3(b), CP3(c), CP3(d), CP3(e), CP3(f), CP3(g), CP3(h), CP3(i). Check the official K327 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Determine density from mass and volume. For an irregular solid that sinks, obtain volume by water displacement and read the meniscus at eye level. Repeat measurements, keep units consistent, and explain how trapped air or an incomplete immersion changes the result.
Exam traps and retrieval check
Avoid these traps
- Treating mass and weight as synonyms.
- Using total surface area instead of contact area.
- Leaving volume in when SI units are required.
Check from memory
Is tension a contact force?
Yes.
What changes in a weaker gravitational field?
Weight decreases while mass stays constant.
How does reducing contact area affect pressure?
It increases pressure for the same normal force.
Official Combined Science scope
This shared Combined Physics owner serves both K326 and K327. Contact and non-contact forces, mass, weight, gravitational field strength, density, and surface pressure.