10_1425_web_Lec_12_WorkandEnergy

# 10_1425_web_Lec_12_WorkandEnergy - Work and Energy Physics...

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Work and Energy Physics 1425 Lecture 12 Michael Fowler, UVa

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What is Work and What Isn’t? In physics, work has a very restricted meaning! Doing homework isn’t work. Carrying somebody a mile on a level road isn’t work… Lifting a stick of butter three feet is work—in fact, about one unit of work.
Work is only done by a force and, the force has to move something! Suppose I lift one kilogram up one meter… I do it at a slow steady speed—my force just balances its weight, let’s say 10 Newtons. Definition: if I push with 1 Newton through 1 meter, I do work 1 Joule. So lifting that kilogram took 10 Joules of work.

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Only motion in the direction of the force counts … Carrying the weight straight across the room at constant height does no work on the weight. After all, it could have been just sliding across on ice—and the ice does no work! What about pushing a box at constant velocity up a frictionless slope?
Pushing a box up a frictionless slope… Suppose we push a box of mass m at a steady speed a distance L up a frictionless slope of angle α . The work done is FL = mgL sin α = mgh where h is the height gained.

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10_1425_web_Lec_12_WorkandEnergy - Work and Energy Physics...

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