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Trouble with "flexure of beams" apparatus



Does anyone have any experience with the old Cenco "beam flexing apparatus"
used to determine Young's modulus? I have never used this before, but now
have a student doing an independent study project based on this procedure.
He is both varying the load while maintaining a constant distance between
the supports, and varying the distance while maintaining a constant load.
Then he plots the appropriate graphs (deflection vs load, and deflection vs.
cube of distance), getting nice straight lines, and using the slopes to find
Young's modulus. But one method is giving results in the 3.3 to 3.8 (times
some power of 10) range while the other gives values in the 1.8 to 2.2
range. I've examined his work and haven't been able to discover any
mistakes. Do you know of any typical trouble spots in this procedure or
have any idea what might be wrong?

______________________________________
Fred Lemmerhirt
Waubonsee Community College
Sugar Grove, Illinois
flemmerhirt@mail.wcc.cc.il.us <mailto:flemmerhirt@mail.wcc.cc.il.us>
http://chat.wcc.cc.il.us/~flemmerh/physics.html
<http://chat.wcc.cc.il.us/~flemmerh/physics.html>