Practice question · Multiple choice
Doubling a car's speed roughly quadruples its braking distance. Which relationship explains that, and why does it matter for speed limits?
Hints
- Write the kinetic energy formula. Which term is squared?
- The brakes do work equal to force times distance. Ask what has to be dissipated.
Show the answer
D. Kinetic energy goes as v², so twice the speed is four times the energy
Why
Energy scales with the square of speed, and the brakes must dissipate all of it as work over the stopping distance, so 30 to 60 km/h is four times the distance, not twice. It is the strongest argument for urban speed limits, and it is entirely contained in the exponent.
Practise Energy and Conservation Laws
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