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WORK 8.2

WORK 8.2. Chapter Eight: Work. 8.1 Work 8.2 Efficiency and Power. Chapter 8.2 Learning Goals. Describe the relationship between work and power. Apply a rule to determine the amount of power required to do work. Explain the meaning of efficiency in terms of input and output work.

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WORK 8.2

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  1. WORK 8.2

  2. Chapter Eight: Work • 8.1 Work • 8.2 Efficiency and Power

  3. Chapter 8.2 Learning Goals • Describe the relationship between work and power. • Apply a rule to determine the amount of power required to do work. • Explain the meaning of efficiency in terms of input and output work.

  4. Key Question: How can a machine multiply forces? Investigation 8B Work

  5. 8.2 Efficiency and Power • Every process that is done by machines can be simplified in terms of work: • work input:the work or energy supplied to the process (or machine). • work output:the work or energy that comes out of the process (or machine).

  6. 8.2 Efficiency and Power • A rope and pulley machine illustrates a rule that is true for all processes that transform energy. • The total energy or work output can never be greater than the total energy or work input.

  7. 8.2 Efficiency • 65% of the energy in gasoline is converted to heat. • As far as moving the car goes, this heat energy is “lost”. • The energy doesn’t vanish, it just does not appear as useful output work.

  8. 8.2 Efficiency • The efficiency of a machine is the ratio of usable output work divided by total input work. • Efficiency is usually expressed in percent. Output work (J) efficiency = Wo Wi x 100% Input work (J)

  9. Solving Problems • You see a newspaper advertisement for a new, highly efficient machine. The machine claims to produce 2,000 joules of output work for every 2,100 joules of input work. • What is the efficiency of this machine? • Is it as efficient as a bicycle? • Do you believe the advertisement’s claim? Why or why not?

  10. Solving Problems • Looking for: • …efficiency of machine • Given: • …Wi = 2100 J, Wo = 2000 J • Relationships: • % efficiency = Wo x 100 Wi • Solution • 2000 J ÷ 2100 J x 100 = 95% efficient

  11. 8.2 Power • The rate at which work is done is called power. • It makes a difference how fast you do work.

  12. 8.2 Power • Michael and Jim do the same amount of work. • Jim’s power is greater because he gets the work done in less time.

  13. 8.2 Power • Power is calculated in watts. • One watt (W) is equal to 1 joule of work per second. • James Watt, a Scottish engineer, invented the steam engine. • Jame Watt explained power as the number of horses his engine could replace. • One horsepower still equals 746 watts.

  14. 8.2 Power Work (joules) P = W t Power (watts) Time (s)

  15. Solving Problems • Allen lifts his weight (500 newtons) up a staircase that is 5 meters high in 30 seconds. • How much power does he use? • How does his power compare with a 100-watt light bulb?

  16. Solving Problems • Looking for: • …power • Given: • Fweight= 500 N; d = 5 m, t = 30 s • Relationships: • W = F x d; P = W ÷ t • Solution • W = 500 N x 5 m = 2500 Nm • P = 2500 Nm ÷ 30 s = 83 watts • Allen’s power is less than a 100-watt light bulb.

  17. Key Question: What’s your work and power as you climb a flight of stairs? Investigation 8C People Power

  18. Human-powered Transportation • When we move our bodies along, whether by walking, swimming, or skiing, we exert forces over a distance and do work.

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