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A popular book of physics puzzles (Mad About Physics, Jargodski & Potter, 2001) states that if a car rolling down a slippery ramp locks its rear wheels only (while not braking the front wheels) it will turn 180° to slide down rear first. The reason given is that because the front wheels are in “static friction” with the surface (because they are rolling, not sliding), they have a greater force on them than exists on the rear wheels, which are in “sliding friction” mode. What is wrong with this argument?   Consider the parallel case of a car rolling on an icy level road. As you may know from driving (or being driven), as long as you don’t accelerate, brake, turn, get hit by wind, etc. your car will continue to run straight – Newton’s First Law of Motion. If you then braked only your rear wheels, what would happen? You would slow down, i.e. accelerate backwards, proportional directly to your rear tires’ load and coefficient of sliding friction, and inversely to... read more

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