Quote:
Mike Sigman wrote:
A screwjack is used to lift up a car, so the screwjack exerts a force greater than the weight of the car. What is the mass associated with the force from the screwjack?
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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.