The crank is an invention almost as old as the wheel. This simple mechanism allows the application of much more force with less effort from the user. This is called mechanical advantage. The main axis of the crank is about 2 inches diameter (a radius of 1 inch). The handle is offset about 6 inches from the main axis. That means that in one revolution of the crank the handle moves 6 times further than the rope on the spindle. In a simple machine with no friction the product of the applied force times the distance moved (input work) is equal to the output force times the distance the rope moves (output work). Thus because the rope moves only 1 foot for every 6 feet the handle moves, the tension in the rope is magnified six times the effort exerted in the crank's handle. The cranking effort is about 20 pounds. This becomes magnified by a factor of six to exert a 120 pound pull on the rope.
homemade crank is part the College View Observatory (under
construction) that is used to close the roll-off roof with
very little effort. The end of the rope is attached to
the opened roof in order to close the roof. The image
at right shows a similar crank at the other end of the
structure that is used to open the roof.
Click on either of the images to see a short video clip
of the crank in action. The College View
Observatory construction may be viewed at its photo site: https://picasaweb.google.com/113788067378510615658
Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us. Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to email@example.com.
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