Physics Photo of the Week
February 2, 2013
Cranky Physics
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.
This
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 dcollins@warren-wilson.edu.
All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion. These photos and discussions may be used for private individual use or educational use. Any commercial use without written permission of the photoprovider is forbidden.
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