Driving dynamics and physics: mass, braking distance, centrifugal force, aquaplaning
Physics questions reward candidates who can reason with two relationships: kinetic energy and braking distance grow with the square of speed, and lateral force grows with the square of speed and inversely with radius. Mass matters less for pure braking distance on a dry road than most candidates think, and far more for stability, rollover risk and downhill behaviour.
Braking distance
Total stopping distance = reaction distance + braking distance. Reaction distance = speed in m/s × reaction time (about 1 s, more when tired). Braking distance ≈ v² / (2 × μ × g), where μ is the friction coefficient - roughly 0.8 on dry asphalt, 0.5 on wet, 0.2–0.3 on snow or ice. Because of the square, doubling speed quadruples braking distance.
Centrifugal force and stability
F = m × v² / r. A high centre of gravity from a badly loaded container, combined with a small radius, is what rolls trucks over on slip roads. Lateral acceleration above roughly 0.4 g is critical for a loaded articulated vehicle.
Aquaplaning
When water cannot be displaced fast enough, the tyre lifts off the road. Risk rises with speed, water depth, worn tread and under-inflation, and falls with load. Front tyres usually lift first, so steering goes light before you feel the rear. Correct response: ease off the accelerator, do not brake hard, keep the wheel straight, and let speed fall.
Worked example
A 40 t combination at 80 km/h (22.2 m/s). Reaction distance at 1 s = 22 m. Braking distance with μ = 0.5: 22.2² / (2 × 0.5 × 9.81) = 493 / 9.81 ≈ 50 m. Total ≈ 72 m. A car at the same speed with μ = 0.8 needs 493 / 15.7 ≈ 31 m plus 22 m reaction = 53 m. The truck needs nearly 20 m more, which is why the gap you leave must be larger than a car's.
Centrifugal check: 40 t at 50 km/h (13.9 m/s) on a 50 m radius curve gives F = 40 000 × 193 / 50 ≈ 154 kN, i.e. about 0.39 g of lateral acceleration - right at the limit.
Traps
Believing a heavier load always shortens or lengthens braking distance rather than destabilising the vehicle; braking hard while aquaplaning; using the engine brake on a slippery surface and locking the drive axle; forgetting that a partial load can shift and change the centre of gravity mid-corner.