Chapter 7 Work Ewen et al 2005 7

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Chapter 7: Work (Ewen et al. 2005) 7. 2 Power Objectives: • Define power

Chapter 7: Work (Ewen et al. 2005) 7. 2 Power Objectives: • Define power and solve power problems

Power �Power is the rate of doing work; how fast work is done. �Units

Power �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

Example 1 �A freight elevator operator weighs 5. 00 E 3 N. If it

Example 1 �A freight elevator operator weighs 5. 00 E 3 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 k. W

Example 2 �The power expended in lifting an 825 lb girder to the top

Example 2 �The power expended in lifting an 825 lb girder to the top of a building 1. 00 E 2 ft high is 10. 0 hp. How much time is required to raise the girder? �Ans: 15. 0 s

Example 3 �The mass of a large steel wrecking ball is 2. 00 E

Example 3 �The mass of a large steel wrecking ball is 2. 00 E 3 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 k. W

Example 4 �A machine is designed to perform a given amount of work in

Example 4 �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: 2 P

Example 5 �A motor is capable of developing 10. 0 k. W of power.

Example 5 �A motor is capable of developing 10. 0 k. W of power. How large a mass can it lift 75. 0 m in 20. 0 s? �Ans: 272 kg

Example 6 �A pump is needed to lift 1. 50 E 3 L of

Example 6 �A pump is needed to lift 1. 50 E 3 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 k. W