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PUZZLER

Solve the Bicycle Problem

Prop up your bicycle so that it will not fall over. Do not apply pressure to the bike in any direction. Rotate the crank so that one of the crank arms is facing straight down, then tie a length of string around that crank arm. It will look something like this:



Now pull back on the string. Does the bike move forward, backward, or stay in the same spot?

Hint: The bike's rear wheel will not skid.

Solution

Counter intuitively, the bike's crank spins the opposite way from the direction you are pulling.

The bike moves backward, and the crank also spins backward. (On a standard freewheel or cassette driven bike, the crank needs to spin backward to allow the rear wheel to spin backward).

When you pull the bike as a whole backward, the rear wheel wants to spin backward from friction with the ground. The crank of a bicycle must always spin backward if the rear wheel is spinning backward.

Watch the video below for a demonstration






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