Edexcel IAL Physics revision · AS — Foundations
Physical quantities and units
The first chapter of AS, and the one that never stops being examined — because units, vectors and uncertainties appear inside every practical paper and most theory questions until the final A2 exam.
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What the syllabus demands
- —Use SI base units and check equations for homogeneity
- —Express values with appropriate significant figures and prefixes (pico to tera)
- —Distinguish scalars and vectors; add vectors by drawing and by calculation
- —Resolve a vector into perpendicular components
- —Distinguish systematic and random errors; combine absolute and percentage uncertainties
Definitions that earn marks
Clear definitions to practise — check your course mark scheme
- Homogeneity
- An equation is homogeneous when every term has the same base units. Homogeneity is necessary for a correct equation, but does not prove it correct.
- Systematic error
- A consistent bias in measurements, such as a zero offset or calibration error. Its size may depend on the measured value; repeating and averaging does not remove it.
- Random error
- Scatter of readings around the true value with no pattern — reduced by repeating and averaging.
- Precision vs accuracy
- Precise readings cluster closely together; accurate readings are close to the true value. A miscalibrated instrument can be precise but not accurate.
The equations
More equations to practise: the Edexcel IAL formula sheet.
Where the marks die
Common mistakes to check
- 01
Averaging away a systematic error. Averaging only reduces random error — a zero error survives every repeat. Say which type the error is before saying how to reduce it.
- 02
Forgetting to multiply percentage uncertainty by the power: if T² appears in your equation, the uncertainty in T is doubled on the way through.
- 03
Resolving with the wrong trig function. The component adjacent to the angle takes cos; opposite takes sin. Draw the triangle every time — guessing costs the mark.
- 04
Quoting an answer to more significant figures than the data. Match the least precise value given — usually 2 or 3 s.f.
One worked example, done properly
Question
A force of 20 N acts at 30° above the horizontal. Find its horizontal and vertical components.
Method
- 1.Horizontal (adjacent to the angle): Fx = 20 cos 30° = 17.3 N.
- 2.Vertical (opposite the angle): Fy = 20 sin 30° = 10 N.
Fx ≈ 17.3 N, Fy = 10 N