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Old 10-01-2008, 08:09 AM   #113
Mike Sigman
Location: Durango, CO
Join Date: Feb 2005
Posts: 4,123
Re: How Long and In What Manner to Great Mastery?

Erick Mead wrote: View Post
In that example, the mass of the operator accelerates in a reciprocating cycle to displace a crank, turning a screw in shear. That external force (torque, actually) is applied to the screwjack, which causes a spiral extension (or retraction). A screw exerts no force, it merely holds an extension, The cranked screw provides a mechanical advantage exceeding the proportional difference in mass. The mechanical advantage of the screw is complex but (disregarding friction and angle of attack) is very roughly a function of the proportional difference in the crank length and the radius and spacing of the threads. Very large effective lever arm -- but it all works by shear.
How about the mass the screwjack is sitting on, though? I.e., in the transfer of forces to the car, the solid connection to the earth plays a role in the mass x acceleration component of the force equation. The mass of the earth is considerable. If you are "straightening out" a force that derives its support from the earth, let's say a punch for example, it affects the whole perspective of F = ma and brings into play some applicable thoughts about Impulse and momentum.


Mike Sigman


Mike Sigman
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