Uploaded December 2020 | Updated September 2026, 8 hours ago

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When you let go of the pen, it falls down in an arc about the
pencil. At the same time, the heavy mass accelerates
downward, reducing the radius of the pen's arc
dramatically. As the pen spirals inward around the pencil, its
velocity increases rapidly (conservation of angular
momentum) and it very quickly winds around the pencil
several times. It takes only three or four windings around
the pencil to stop the cup from falling any further. The
friction between the string and pencil increases
exponentially with the number of windings.