01
Momentum is a vector — sign the rebound. +30 to −20 m/s is a change of 50, not 10. Set a + direction, sign each velocity, then subtract. Every series
02
Snell's law: take the sines. n = sin i / sin r — not i/r, not from the surface. Also n = 1 / sin c. Every June
03
Background count: subtract, halve, add back. Subtract background first; halve per half-life; add background back if the question wants total rate. Most-flagged
04
Wire resistance ∝ 1/d². R ∝ L/A and A ∝ d². Double the diameter ⇒ R × 1/4, not 1/2. Go via area. Recurring
05
Boyle's law: pV = constant (inverse). Double the volume ⇒ pressure halves. Inversely, not directly, proportional. Recurring
06
Transformer core carries flux, not current. Needs a.c. (DC = no changing flux). a.c.→d.c. is a diode, not a transformer. IₚVₚ = IₛVₛ. Several series
07
Lenz: the induced effect opposes the change. Right-hand rule = generator; left-hand = motor. Max EMF when flux changes fastest. Recurring
08
Beta = an electron from the nucleus. n → p + e, not an outer-shell electron. β−: A same, Z+1. α: A−4, Z−2 (neutrons −2, not −4). Recent series
09
Mass on a spring: resultant = T − W. Use tension minus weight, not tension alone — gravity is still acting. Seen 2025
10
Unit prefixes in E=VIt, P=IV, Q=It. mA→A and min→s, often both in one question. The classic factor-of-1000-or-60 slip. Recurring
Set a + direction; rebound velocity is negative
Change in momentum = (final mv) − (initial mv), signed; impulse = Ft = Δ(mv)
p = F/A; p = ρgh; atmospheric − Δp at height
Floating: compare densities, not masses
Boyle: pV constant (inverse); gas KE depends ONLY on T
Specific heat capacity per kg per °C; thermal capacity = whole object
Evaporation cools the remaining liquid
Snell: n = sin i / sin r; also n = 1/sin c; angle from the normal
TIR: dense → less dense, above critical angle
Frequency unchanged in refraction; ultrasound is longitudinal
Diffraction: longer λ → more; amplitude irrelevant
Converging lens: real enlarged image needs object between f and 2f
R = V/I (not gradient for non-ohmic); R ∝ L/A, not L/d
Thermistor T↑ → R↓; diode = one-way; NAND + inverters → OR
EMF induced by a changing field; faster motion → larger EMF
Transformer core = flux not current; needs a.c.; diode makes d.c.
Lenz opposes the change; left = motor, right = generator
α: A−4, Z−2; β−: A same, Z+1; γ: A,Z same
Background: subtract before halving, add back at the end
Fusion = Sun; fission = power station; Rutherford: small dense + nucleus
Hubble v = H₀d (direct); orbital speed = 2πr/T
1 light-year ≈ 9.5 × 10^15 m