Friction, inclined plane.

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masterflex
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Homework Statement


This is a conceptual/logic question I have. I don't understand why for minimum P: P must be perpendicular to R; angle beta = psi_s. (please see attached picture). Once I can get my arms around that statement, I can solve the problem. Thanks for any logic help. Or if there's another way to approach this, that's cool too.


Homework Equations





The Attempt at a Solution


Why is the angle Beta not = 0? That is the most puzzling thing conceptually. It would seem that if angle Beta=0, you would get the most mechanical efficiency because the direction of force P would then be perpendicular to the normal N.
 

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