Wire it up.
Plot the I–V graph of a resistor, a filament lamp and a diode, stretch a wave until v = fλ makes sense, and stop a car on ice.
Three components, three very different graphs
FIG. 01 · ELECTRICITY · 4.2.1.4A fixed resistor at constant temperature: current is directly proportional to p.d.
Negative values mean the cell has been reversed.
Sets the resistor's value, or the lamp's resistance while the filament is still cold.
In the exam: this is the required practical, and the three shapes are worth learning as pictures. A straight line through the origin means resistance is constant — an ohmic conductor, I ∝ V. The filament lamp curves over because the current heats the filament, the ions vibrate more, and resistance rises. The diode is almost flat one way round and rises steeply the other: it has very high resistance in the reverse direction. Always say at constant temperature when you quote Ohm's law.
The wave moves. The water does not.
FIG. 02 · WAVES · 4.6.1.2Waves passing a point each second. It is set by the source and does not change when the wave enters a new material.
Crest to crest — the marked pair of dashed lines on the diagram.
The single marked particle only ever moves up and down: that is what makes this wave transverse.
In the exam: v = fλ appears constantly, usually with one quantity hidden inside a unit conversion (cm to m, kHz to Hz). Two ideas earn the extra marks: the wave transfers energy without transferring matter, and in a transverse wave the oscillations are perpendicular to the direction of energy transfer, while in a longitudinal wave they are parallel to it.
Double the speed, quadruple the braking distance
FIG. 03 · FORCES & BRAKING · 4.5.630 mph is about 13 m/s; the motorway limit is about 31 m/s.
Tiredness, alcohol, drugs and distractions all push this up — and only this.
Good grip: the braking force can decelerate the car at about 6.5 m/s².
In the exam: stopping distance = thinking distance + braking distance. Thinking distance is just speed × reaction time, so it doubles when the speed doubles. Braking distance comes from the work done by the braking force, Fd = ½mv², so d = v²/(2a) — it quadruples when the speed doubles. Wet or icy roads and worn tyres or brakes change the braking distance only; tiredness, alcohol and phones change the thinking distance only. Larger braking forces also mean greater deceleration, more heat in the brakes and a higher risk of skidding.