Flashcards · 9702 · AS Topic 1

Physical quantities and units (AS Level) flashcards

22 cards on AS Topic 1, subtopics 1.1 to 1.4. Pick a subtopic or study them all.

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What two parts must every physical quantity have?: A numerical magnitude and a unit. "20" means nothing until it is 20 m, 20 s or 20 N.
How many significant figures should a calculated answer have?: No more than the data justify: if the least precise measurement has 2 significant figures, the answer should not claim 5.
Estimate the speed of sound in air and the speed of light in a vacuum: Sound in air: about 340 m s⁻¹. Light in a vacuum: 3.00 × 10⁸ m s⁻¹.
Estimate the wavelength range of visible light: About 400 nm (violet) to 700 nm (red), i.e. 4 × 10⁻⁷ m to 7 × 10⁻⁷ m.
Order-of-magnitude estimate: the diameter of an atom: About 10⁻¹⁰ m.

The five SI base quantities and units used in 9702: Mass: kilogram (kg). Length: metre (m). Time: second (s). Electric current: ampere (A). Temperature: kelvin (K).
Express the newton and the joule in SI base units: N = kg m s⁻² (from F = ma). J = kg m² s⁻² (from W = Fs).
Express the watt and the pascal in SI base units: W = kg m² s⁻³ (J s⁻¹). Pa = kg m⁻¹ s⁻² (N m⁻²).
Express the volt in SI base units: V = J C⁻¹ = J (A s)⁻¹ = kg m² s⁻³ A⁻¹.
Homogeneity of an equation: what it is, and its limit: Both sides must reduce to the same base units. It shows an equation could be right, not that it is: it cannot detect a wrong dimensionless constant (s = ut + at² is homogeneous but wrong).
Prefixes: T, G, M, k, d, c, m, μ, n, p: T 10¹², G 10⁹, M 10⁶, k 10³, d 10⁻¹, c 10⁻², m 10⁻³, μ 10⁻⁶, n 10⁻⁹, p 10⁻¹².

Random error: Readings scatter unpredictably above and below the true value (for example, reaction time with a stopwatch). Reduced by repeating and averaging.
Systematic error: A consistent offset in every reading (for example, a ruler with a worn end). Not reduced by repeating; only by recalibration or zeroing.
Zero error: The instrument does not read zero when it should: a specific example of a systematic error.
Precision vs accuracy: Precise: repeated readings agree closely (small random error). Accurate: close to the true value (small systematic error). Readings can be precise but not accurate.
How do uncertainties combine when quantities are added or subtracted, multiplied or divided, or raised to a power n?: Add or subtract: add absolute uncertainties. Multiply or divide: add percentage uncertainties. Power n: multiply the percentage uncertainty by n.
A sphere has radius (2.0 ± 0.1) cm. What is the percentage uncertainty in its volume?: % uncertainty in r = 0.1 ÷ 2.0 × 100 = 5%. V ∝ r³, so 3 × 5% = 15%.

Scalar vs vector, with examples: Scalar: magnitude only (distance, speed, mass, time, energy, temperature). Vector: magnitude and direction (displacement, velocity, acceleration, force, momentum).
Components of a force F at angle θ to the horizontal: Horizontal: F cos θ. Vertical: F sin θ. cos θ is always the component along the reference direction the angle is measured from.
Forces of 3.0 N and 4.0 N act at right angles. Find the resultant.: √(3.0² + 4.0²) = 5.0 N, at tan⁻¹(4.0/3.0) = 53° to the 3.0 N force.
Two ways to add coplanar vectors: Draw them tip-to-tail in a scale vector diagram, or resolve each into perpendicular components, add the components, then recombine.
How do you subtract one vector from another?: Add its negative: A − B = A + (−B), where −B has the same magnitude as B but the opposite direction.