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Work & Energy Ideas: ch 11 Work either adds or subtracts energy. We’ll only deal with mechanical work (not heat) Work & Energy Energy Transfer: Mechanical means: pull, push, etc. Heat: not covered Work: positive or negative? W>0 environment does work on the system and energy goes up. W<0 system does work on the environment and energy goes down Energy & Work Work = F x Distance , it’s really the amount of force acting in the direction the object is moving, or W = FxcosΘ When there is a variable force, then the Work done is the area under the Force vs. distance curve W = Fdx Energy & Work Special Case: what about a force acting perpendicular to the direction of motion. This is “just like” circular motion where a net centripetal force is acting. How much work does the centripetal force do? Energy & Work Potential Energy and conservative forces We call gravity a conservative force. The work done by gravity is independent of the path taken…..it only depends on the starting and ending positions. Spring force/energy is also conservative, does not depend on the path, only the starting and ending postions Frictional force is not conservative: the work done by friction does depend on the path. Energy & Work Finding force from Potential Energy W = F Δx = -U (work = -change in potential energy) U = -FΔx Or…F = -ΔU/Δx ….this is the slope of the U vs. x graph