For some time now we’ve been analyzing the helpfulness of the engineering phenomena known as between moving parts. Like an unsummoned gremlin, friction will be standing by in any mechanical situation to put the wrench in the works. Today we’ll calculate just friction within the example how much friction is presentwe’ve been working with. compound pulley
Last time we began our numerical demonstration of the inequality between a compound pulley’s work input, F, of 10 pounds to extract a length of rope, d, of 8 feet._{2} In reality our compound pulley must contend with the effects of friction, so we know it will take more than 10 pounds of force to lift the urn, a resistance which we’ll notate F, represented by the formula,_{Actual}
F + F(1)_{F }We find that 12 which simplifies to, 2 Now that we’ve determined values for all operating variables, we can solve for work input and then contrast our finding with work output,
+ (F _{F}× d) (4)_{2}
We previously calculated work output,
It’s evident that the amount of work Mr. Toga puts into lifting his urn requires 16 more Foot-Pounds of work input effort than the amount of work output produced. This extra effort that’s required to overcome the pulley’s friction is the same as the work required to carry a weight of one pound a distance of 16 feet. We can thus conclude that work input does not equal work output in a Next time we’ll take a look at a different use for pulleys beyond that of just lifting objects. Copyright 2017 – Philip J. O’Keefe, PE Engineering Expert Witness Blog ____________________________________ |