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WORK 8.2. Chapter Eight: Work  8.1 Work  8.2 Efficiency and Power.

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Presentation on theme: "WORK 8.2. Chapter Eight: Work  8.1 Work  8.2 Efficiency and Power."— Presentation transcript:

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

5 8.2 Efficiency and Power  Every process that is done by machines can be simplified in terms of work: 1.work input: the work or energy supplied to the process (or machine). 2.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. efficiency = W o W i Output work (J) Input work (J) x 100%

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10  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? Solving Problems

11 1.Looking for:  …efficiency of machine 2.Given:  …W i = 2100 J, W o = 2000 J 3.Relationships:  % efficiency = W o x 100 W i 4.Solution  2000 J ÷ 2100 J x 100 = 95% efficient Solving Problems

12 8.2 Power  The rate at which work is done is called power.  It makes a difference how fast you do work.

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14 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.

15 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.

16 8.2 Power P = W t Time (s) Work (joules) Power (watts)

17  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? Solving Problems

18 1.Looking for:  …power 2.Given:  F weight = 500 N; d = 5 m, t = 30 s 3.Relationships:  W = F x d; P = W ÷ t 4.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. Solving Problems

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

21 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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