OCR A Level Physics: Foundations of Physics — Revision Notes

Condensed for the final weeks. For the full explanation, use the Foundations of Physics study guide.

Physical quantities and units (2.1)

Six of the seven SI base quantities examined here (SI itself defines seven; this specification excludes luminous intensity): mass (kg), length (m), time (s), current (A), temperature (K), amount of substance (mol). Derived units built from these — e.g. momentum (kg m s⁻¹), density (kg m⁻³).

Homogeneity check: verify an equation’s units balance on both sides before treating it as correct — a dimensionally inconsistent equation cannot be physically correct.

Prefixes: pico to tera. Graph/table convention: label as “quantity / unit,” e.g. “speed / m s⁻¹”.

Making measurements and analysing data (2.2)

Error type	Cause	Fix
Systematic (incl. zero error)	Consistent bias in every reading	Recalibrate instrument
Random	Unpredictable variation	Repeat and average

Precision vs. accuracy: precise = results closely clustered together; accurate = close to the true value. A thermometer with a zero error gives precise but consistently inaccurate readings — a systematic error, not random.

Combining by ADDITION/SUBTRACTION: add ABSOLUTE uncertainties
Combining by MULTIPLICATION/DIVISION: add PERCENTAGE uncertainties

Graphical treatment: lines of best fit, worst lines, percentage difference; elementary error bars expected at A Level.

Nature of quantities: scalars and vectors (2.3)

	Has direction?
Scalar	No (e.g. speed, mass, energy)
Vector	Yes (e.g. velocity, force, displacement)

Resolving a vector into perpendicular components:

Fx = F cos(theta)   [component ALONG the axis from which theta is measured]
Fy = F sin(theta)   [component PERPENDICULAR to that axis]

The most common error in mechanics: swapping sin and cos when resolving.

Worked example: resolving a vector

A force of 40 N acts at 30° above the horizontal.

Fx = 40 x cos(30) = 40 x 0.866 = 34.6 N
Fy = 40 x sin(30) = 40 x 0.5 = 20 N

This exact method reappears throughout the course whenever an object moves or is acted on at an angle.

Worked example: combining percentage uncertainties

Length = 12.0 cm ± 0.1 cm; time = 4.0 s ± 0.1 s. Find the percentage uncertainty in speed = length ÷ time.

% uncertainty in length = (0.1 / 12.0) x 100 = 0.83%
% uncertainty in time   = (0.1 / 4.0) x 100 = 2.5%

Since speed = length / time (division), ADD percentage uncertainties:
Total % uncertainty = 0.83% + 2.5% = 3.33%

Worked example: finding the resultant of two vectors

Two forces act on an object: 30 N due east and 40 N due north. Find the resultant force’s magnitude and direction.

Since the two forces are perpendicular, use Pythagoras:
Resultant magnitude = sqrt(30^2 + 40^2) = sqrt(900 + 1600)
                     = sqrt(2500) = 50 N

Direction (angle from east, measured toward north):
tan(theta) = opposite / adjacent = 40 / 30
theta = tan^-1(40/30) = 53.1 degrees

For two forces that are NOT perpendicular, a scale drawing (a vector triangle) or resolving both into perpendicular components first is the method the specification names – practise both approaches, since exam questions may specifically ask for either a calculation method or a scale-drawing method.

Why estimating quantities matters

The specification explicitly expects candidates to make sensible estimates of physical quantities – for example, the mass of an apple, the height of a room, or the speed of a walking person – as part of 2.1.1. This is not a minor add-on: being able to sanity-check a calculated answer against a sensible real-world estimate (does 500,000 m/s for a car’s speed look obviously wrong?) is a genuine physics skill the specification tests directly, separate from being able to perform the calculation itself correctly.

Key terms

Systematic error — a consistent bias affecting every reading the same way (e.g. a zero error). Random error — unpredictable variation between repeated readings. Precision — how closely clustered repeated results are. Accuracy — how close results are to the true value. Scalar — a quantity with magnitude only. Vector — a quantity with magnitude AND direction.

Common mistakes

• Confusing precision (clustered) with accuracy (close to true value).
• Adding absolute uncertainties instead of percentage uncertainties when combining by multiplication/division, or vice versa for addition/subtraction.
• Swapping sin and cos when resolving a vector — cos for the component along the axis from which the angle is measured, sin for the component perpendicular to that axis.
• Treating a scalar as if it had direction (or vice versa) — speed (scalar) vs. velocity (vector) is the classic test case.
• Forgetting to check unit homogeneity before accepting a derived formula.

Why Module 2 resurfaces silently

Every later module assumes SI units, uncertainty handling and the vector-scalar distinction without re-teaching them: Module 3’s kinematics, Module 5’s astrophysics, Module 6’s particle physics all rely on skills introduced here. Gaps in Module 2 tend to show up as recurring errors throughout the A Level, not isolated Module 2 mistakes — over-learn this module rather than revising it once.

Quick self-test

• Resolve a 60 N force acting at 45° into horizontal and vertical components.
• A length is 8.0 cm ± 0.2 cm; find its percentage uncertainty.
• Explain the difference between a systematic and a random error, with an example of each.
• State which is the scalar and which is the vector: speed/velocity, mass/weight, distance/displacement.
• Explain why an equation that fails a unit-homogeneity check cannot be correct.

Related resources

• Foundations of Physics study guide
• Foundations of Physics practice questions

Official syllabus

OCR, A Level GCE Physics A H556 Specification, version 3.0 (March 2026), Module 2: Foundations of physics, https://www.ocr.org.uk/images/171726-specification-accredited-a-level-gce-physics-a-h556.pdf, fetched and verified in full 2026-09-02.