2 Chapter Eight: Work8.1 Work8.2 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 8BWorkKey Question:How can a machine multiply forces?
5 8.2 Efficiency and PowerEvery 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 PowerA 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 efficiency = Wo Wi 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 = WoWix 100%Input work (J)
10 Solving ProblemsYou 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?
11 Looking for: Given: Relationships: Solution Solving Problems …efficiency of machineGiven:…Wi = 2100 J, Wo = 2000 JRelationships:% efficiency = Wo x 100WiSolution2000 J ÷ 2100 J x 100 = 95% efficient
12 8.2 Power The rate at which work is done is called power. It makes a difference how fast you do work.
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 PowerWork (joules)Power (watts)P = WtTime (s)
17 Solving ProblemsAllen 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?
18 Solving Problems Looking for: Given: Relationships: Solution …power Fweight= 500 N; d = 5 m, t = 30 sRelationships:W = F x d; P = W ÷ tSolutionW = 500 N x 5 m = 2500 NmP = 2500 Nm ÷ 30 s = 83 wattsAllen’s power is less than a 100-watt light bulb.