About The braking-distance rule of thumb: (speed:10)² : 2: which statement is WRONG?
- ✓ If speed doubles, braking distance also doubles (a linear relationship).
- The rule of thumb literally is '(speed(km/h):10) x (speed(km/h):10) : 2', with the book's own worked examples 50 km/h = 12.50 m, 80 km/h = 32.00 m, 130 km/h = 84.50 m.
- Stopping distance = reaction distance + braking distance.
Correct answer: If speed doubles, braking distance also doubles (a linear relationship).
Explanation
'braking distance increases progressively: if you drive 2x as fast, your braking distance becomes 4x as long.' At 40 km/h braking distance is 8.00 m; at 80 km/h (2x as fast) it is 32.00 m — 4x, not double
Why learners get this wrong
- The rule of thumb literally is '(speed(km/h):10) x (speed(km/h):10) : 2', with the book's own worked examples 50 km/h = 12.50 m, 80 km/h = 32.00 m, 130 km/h = 84.50 m. Without the :2, every figure is exactly double the book value
- Stopping distance = reaction distance + braking distance. At 50 km/h: 13.75 m (reaction distance) + 12.50 m (braking distance) = 26.25 m stopping distance, more than double the braking distance alone
Topic: The braking-distance rule of thumb: (speed:10)² : 2
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