A driver’s manual states that the stopping distance

quadruples as the speed doubles; that is, if it takes

30 ft to stop a car moving at 25 mph then it would

take 120 ft to stop a car moving at 50 mph. Justify

this statement by using mechanics and the fi rst law

of thermodynamics. [Assume that when a car is

stopped, its kinetic energy (1


) is totally converted to heat.]

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