WORK 8.2

WORK 8.2

Chapter Eight: Work

8.1 Work8.2 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.

Investigation 8B

Key Question:How can a machine

multiply forces?

Work

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).

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.

8.2 Efficiency65% 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.

8.2 EfficiencyThe efficiency of a machine is the ratio of usable output work divided by total input work.

Efficiency is usually expressed in percent.

efficiency = Wo Wi

Output work (J)

Input work (J)

x 100%

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

1. Looking for: …efficiency of machine

2. Given: …Wi = 2100 J, Wo = 2000 J

3. Relationships: % efficiency = Wo x 100

Wi 4. Solution 2000 J ÷ 2100 J x 100 = 95% efficient

Solving Problems

8.2 PowerThe rate at which work is done is called power.

It makes a difference how fast you do work.

8.2 PowerMichael and Jim do the same amount of work.

Jim’s power is greater because he gets the work done in less time.

8.2 PowerPower 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.

8.2 Power

P = W t Time (s)

Work (joules)

Power (watts)

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

1. Looking for: …power

2. Given: Fweight= 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 Nm P = 2500 Nm ÷ 30 s = 83 watts Allen’s power is less than a 100-watt light bulb.

Solving Problems

Investigation 8C

Key Question:What’s your work and

power as you climb a flight of stairs?

People Power

Human-powered TransportationWhen we move our bodies along, whether

by walking, swimming, or skiing, we exert forces over a distance and do work.