Force and pressure
Contact and non-contact forces, mass and weight, gravitational fields, density and pressure.
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Before you begin
A force is a push or a pull, and some forces act without anything touching. The lessons start by sorting forces into contact and non-contact forces and telling mass apart from weight, which depends on the gravitational field. They then calculate density from mass and volume, and pressure from a force and the area it acts on.
Be comfortable with: physical quantities and units, because density and pressure calculations need consistent units.
Learning goals
- Distinguish contact forces from non-contact forces
- State that mass measures the amount of matter in a body
- Describe a gravitational field as a region where a mass experiences gravitational force
- Define gravitational field strength as gravitational force per unit mass
- Apply weight = mass × gravitational field strength
- Distinguish mass from weight
- Apply density = mass ÷ volume
- Define pressure as force per unit area
- Apply pressure = force ÷ area
Syllabus statements covered
- identify and distinguish between contact forces (e.g. friction, air resistance, tension and normal force) and non-contact forces (e.g. gravitational, electrostatic and magnetic forces)
- state that mass is a measure of the amount of matter in a body
- state that a gravitational field is a region in which a mass experiences a force due to gravitational attraction
- define gravitational field strength, g, as gravitational force per unit mass placed at that point
- recall and apply the relationship weight = mass × gravitational field strength to new situations or to solve related problems
- distinguish between mass and weight
- recall and apply the relationship density = mass / volume to new situations or to solve related problems
- define pressure in terms of force and area
- recall and apply the relationship pressure = force / area to new situations or to solve related problems
Lessons
Work through them in order.
- Forces, mass and weightClassify forces as contact or non-contact, and use W = mg to tell mass from weight in any gravitational field.
- Density of solids and liquidsCalculate density from mass and volume, convert between g/cm³ and kg/m³, and compare a measured density with reference values.
- Pressure, force and areaCalculate pressure from the perpendicular force and the contact area, and explain designs that raise or lower pressure.
Practise and check
Or choose
Topic reference
Quick reference
| Quantity | Relationship | Unit |
|---|---|---|
| weight | weight = mass × gravitational field strength, W = mg | newton, N |
| gravitational field strength | g = 10 N/kg near the Earth’s surface | N/kg |
| density | density = mass ÷ volume | kg/m³ or g/cm³ |
| pressure | pressure = force ÷ area | pascal, Pa (N/m²) |
| Contact forces | Non-contact forces |
|---|---|
| friction, air resistance, tension, normal force | gravitational, electrostatic, magnetic |
Core knowledge to remember
- Mass is a measure of the amount of matter in a body. It does not depend on where the body is.
- A gravitational field is a region in which a mass experiences a force due to gravitational attraction.
- Gravitational field strength, g, is the gravitational force per unit mass placed at that point.
- Weight is a force: weight = mass × gravitational field strength.
- Pressure is the force acting per unit area.
Common mistakes
- Giving weight in kilograms. Weight is a force, measured in newtons.
- Saying an object’s mass changes on the Moon. Its weight changes; its mass does not.
- Mixing g/cm³ and kg/m³, or cm² and m², in one calculation.
- Saying a larger area gives a larger pressure. The same force over a larger area gives a smaller pressure.