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52 Physical Science Math Skills and Problem Solving Workbook

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Chapter 14 Work, Power, and Machines

Section 14.1 Work and Power(pages 412–416)

Work and Power

Content and Vocabulary Support

What Is Work?Work is the product of force and distance, or:

Work � Force � Distance

Work is measured in newton-meters (N·m), which are called joules (J).

What Is Power?Power is the rate of doing work. Doing work at a faster rate requiresmore power. With more power, you can do more work in the sametime or do the same amount of work in less time. For example, if youuse a snowblower instead of a shovel, you can move more snow inthe same amount of time, or you can move the same amount of snowin a shorter time. This is because the snowblower has more power.

Calculating PowerTo calculate power, you divide the amount of work done by the timeneeded to do the work. The equation for power is:

Power �

Time is measured in seconds (s). The unit of power is the watt (W). Awatt equals one joule per second. For example, an 800-wattmicrowave uses 800 joules of power in one second.

Another common unit of power is the horsepower (hp). Onehorsepower equals about 746 watts. The horsepower of a lawnmowermight be 4 hp, whereas the horsepower of a car might be 190 hp, oralmost 142,000 watts of power.

WorkTime

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Section 14.1 Work and Power

Solved Examples

Example 1: It takes 40 watts of power to operate a light bulb for 1 second. How much work does the light bulb do inthat time?

Given: Power � 40 WTime � 1 s

Unknown: Work

Equation: Power �

Solution: Solve the power equation for work:Work � Power � Time

Substitute the given values:

Work � � � 40 J

Example 2: How much power is needed to lift a book to a height of2.0 meters in a time of 1.0 second, if the book exerts aforce of 20 newtons?

Given: Distance � 2.0 mTime � 1.0 sForce � 20 N

Unknown: Power

Equations: Power � and Work � Force � Distance

Solution: Find the value for work by substituting thegiven values for force and distance in thework equation:

Work � 20 N � 2.0 m � 40 N·m � 40 J

Substitute the values for work and time in thepower equation to find the value for power:

Power � � 40 J/s � 40 W40J1 s

WorkTime

40 J>s1 s

40 W1 s

WorkTime

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Practice Exercises

Exercise 1: A light bulb does 100 joules of work in 2.5 seconds.How much power does it have?

Exercise 2: A man exerts 700 newtons of force to move a piece offurniture 4 meters. If it takes him 2 seconds to move thefurniture, how much power does it require?

Exercise 3: A hair curling iron uses 13 watts of power. How longmust it operate to do 1,040 joules of work?

Exercise 4: A 4-watt night light is left on for 8 hours each night.How much work does it do per night?

Exercise 5: How much horsepower is needed to move a box thathas a force of 1,492 newtons a distance of 2 meters in 1 second?

54 Physical Science Math Skills and Problem Solving Workbook

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54 Physical Science Math Skills and Problem Solving Workbook