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Chapter 7: Work (Ewen et al. 2005) Objectives: Define power and solve power problems Define power and solve power problems

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Power is the rate of doing work; how fast work is done. Units for work? 1 Watt (W) = 1 J/s ft lb/s 1 horsepower (hp) = 550 ft lb/s = 33 000 ft lb/mi

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A freight elevator operator weighs 5.00 E3 N. If it is raised to a height of 15.0 m in 10.0 s, how much power is developed? Ans: 7500 Nm/s, or 7500 J/s, or 7500 W, or 7.50 kW

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The power expended in lifting an 825 lb girder to the top of a building 1.00 E2 ft high is 10.0 hp. How much time is required to raise the girder? Ans: 15.0 s

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The mass of a large steel wrecking ball is 2.00 E3 kg. What power is used to raise it to a height of 40.0 m if the work is done in 20.0 s. Ans: 39200 W or 39.2 kW

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A machine is designed to perform a given amount of work in a given amount of time. A second machine does the same amount of work in half the time. Find the power of the second machine compared with the first. Ans: 2P

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A motor is capable of developing 10.0 kW of power. How large a mass can it lift 75.0 m in 20.0 s? Ans: 272 kg

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A pump is needed to lift 1.50 E3 L of water per minute a distance of 45.0 m. What power, in KW, must the pump be able to deliver? (1 L of water = 1 kg of water) Ans: 11.0 kW

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