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**Work, Power, and Machines**

Chapter 13

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**Work done when a force acts on an object**

AND it moves in the direction of that force Work = force x distance W = Fd work is 0 when an object isn’t moving measured in joules (J)

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**Practice Problem: Work**

A father plays with his daughter by lifting her repeatedly in the air. How much work does he do with each lift if he lifts her 2.0m and exerts an average force of 190 N? Given: Equation: Solution: F = 190N W = Fd W = 190N x 2.0m d= 2.0m W = 380 N ∙ m W =? W = 380 J (N ∙ m = J)

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Power rate at which work is done (how much is done in a given amount of time) Power = work time P = W t measured in watts (W) ! don’t confuse W for work with W for watt !! 1 watt = 1 joule/second

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**Practice Problem: Power**

Lifting an alevator 18m takes 100 kj. If doing so takes 20s, what is the average power of the elevator during the process? Given: Equation: Solution: W = 100kJ 1000J P = W P = 1 x 10 ⁵J 1 kJ t s t = 20s P = 5 x 10³J/s or P = ? kw

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