Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Work, Power, and Machines Objectives Define work and power. Calculate the work done on an object and the rate at which work is done. Use the concept of mechanical advantage to explain how machines make doing work easier. Calculate the mechanical advantage of various machines. Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Work, Power, and Machines Bellringer In your study of motion, you have learned that forces can cause motion. But in some cases, a force that is applied is balanced by another opposite force, and there is no net motion as a result. Look at the following illustrations, and identify the forces and motion in each one. (See illustrations on following slide.) 1. In one drawing, no motion is likely to occur. Which drawing is it? 2. Describe the forces that are acting in this diagram. If the person exerts slightly more force, what happens to the opposite force? Does it increase to match the new force of the person, stay the same, or decrease? Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Bellringer, continued Section 1 Work, Power, and Machines Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Work, Power, and Machines What is Work? Work is the transfer of energy to a body by the application of a force that causes the body to move in the direction of the force. Work is done only when a force causes an object to move in the direction of the force. This is different from the everyday meaning of work. Work Equation work = force  distance W = F  d Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Work, Power, and Machines What is Work?, continued Work is measured in joules. Because work is calculated as force times distance, it is measured in units of newtons times meters, Nm. These units are also called joules (J). In terms of SI base units, a joule is equivalent to 1 kgm 2 /s 2. Definition of joules 1 J = 1 Nm = 1 kgm 2 /s 2 Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Work, Power, and Machines Math Skills Work Imagine a father playing with his daughter by lifting her repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N? 1. List the given and unknown values. Chapter 12 Given:force, F = 190 N distance, d = 2.0 m Unknown:work, W = ? J

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Work, Power, and Machines Math Skills, continued 2. Write the equation for work. Chapter 12 work = force  distance W = f  d 3. Insert the known values into the equation, and solve. W = 190 N  2.0 m = 380 Nm W = 380 J

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Work, Power, and Machines Power Power is a quantity that measures the rate at which work is done or energy is transformed. Power Equation Power is measured in watts. A watt (W) is equal to a joule per second (1 J/s). Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Work, Power, and Machines Math Skills Power It takes 100 kJ of work to lift an elevator 18 m. If this is done in 20 s, what is the average power of the elevator during the process? 1. List the given and unknown values. Chapter 12 Given:work, W = 100 kJ = 1  10 5 J time, t = 20 s The distance of 18 m will not be needed to calculate power. Unknown:power, P = ? W

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Work, Power, and Machines Math Skills, continued 3. Insert the known values into the equation, and solve. 2. Write the equation for power. Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Work, Power, and Machines Machines and Mechanical Advantage Machines multiply and redirect forces. Machines help people by redistributing the work put into them. They can change either the size or the direction of the input force. Different forces can do the same amount of work. A machine allows the same amount of work to be done by either decreasing the distance while increasing the force or by decreasing the force while increasing the distance. Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Force and Work Section 1 Work, Power, and Machines Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Work, Power, and Machines Machines and Mechanical Advantage, continued Mechanical advantage tells how much a machine multiplies force or increases distance. Mechanical Advantage Equation Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Work, Power, and Machines Math Skills Mechanical Advantage Calculate the mechanical advantage of a ramp that is 5.0 m long and 1.5 m high. 1. List the given and unknown values. Chapter 12 Given:input distance = 5.0 m output distance = 1.5 m Unknown:mechanical advantage = ?

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Work, Power, and Machines Math Skills, continued 2. Write the equation for mechanical advantage. Because the information we are given involves only distance, we only need part of the full equation: 3. Insert the known values into the equation, and solve. Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 Simple Machines Objectives Name and describe the six types of simple machines. Discuss the mechanical advantage of different types of simple machines. Recognize simple machines within compound machines. Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The Lever Family The most basic machines are called simple machines. The six types of simple machines are divided into two families. The lever family:The inclined plane family: simple lever simple inclined plane pulley wedge wheel and axle screw Section 2 Simple Machines Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The Lever Family, continued Levers have a rigid arm and a fulcrum. Levers are divided into three classes. All first-class levers have a fulcrum located between the points of application of the input and output forces. In a second-class lever, the fulcrum is at one end of the arm and the input force is applied to the other end. Third-class levers multiply distance rather than force. As a result, they have a mechanical advantage of less than 1. Section 2 Simple Machines Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Levers Section 2 Simple Machines Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The Lever Family, continued Pulleys are modified levers. The point in the middle of a pulley is like the fulcrum of a lever. A single, fixed pulley has a mechanical advantage of 1. Multiple pulleys are sometimes put together in a single unit called a block and tackle. A wheel and axle is a lever or pulley connected to a shaft. The steering wheel of a car, screwdrivers, and cranks are common wheel-and-axel machines. Section 2 Simple Machines Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Pulleys Section 2 Simple Machines Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The Inclined Plane Family Inclined planes multiply and redirect force. An inclined plane turns a small input force into a large output force by spreading the work out over a large distance. A wedge is a modified inclined plane. A screw is an inclined plane wrapped around a cylinder. Section 2 Simple Machines Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Compound Machines A machine made of more than one simple machine is called a compound machine. Examples of compound machines are: scissors, which use two first class levers joined at a common fulcrum a car jack, which uses a lever in combination with a large screw Section 2 Simple Machines Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 4 Conservation of Energy Objectives Analyze the efficiency of machines. Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 4 Conservation of Energy Efficiency of Machines Not all of the work done by a machine is useful work. A machine cannot do more work than the work required to operate the machine. Because of friction, the work output of a machine is always somewhat less than the work input. Efficiency is the ratio of useful work out to work in. Efficiency is usually expressed as a percentage. The efficiency of a machine is a measure of how much useful work it can do. Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 4 Conservation of Energy Efficiency of Machines, continued Efficiency Equation Perpetual motion machines are impossible. Energy is always lost to friction or air resistance. Machines need energy input. Because energy always leaks out of a system, every machine needs at least a small amount of energy input to keep going. Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 4 Conservation of Energy Math Skills Efficiency A sailor uses a rope and an old, squeaky pulley to raise a sail that weighs 140 N. He finds that he must do 180 J of work on the rope in order to raise the sail by 1 m (doing 140 J of work on the sail). What is the efficiency of the pulley? Express your answer as a percentage. 1. List the given and unknown values. Given:work input = 180 J useful work output = 140 J Unknown:efficiency = ? % Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 4 Conservation of Energy Math Skills, continued 2. Write the equation for efficiency. 3. Insert the known values into the equation, and solve. Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Understanding Concepts, continued 4. A coal-burning power plant produces electrical energy with an efficiency of 30%. If the chemical energy produced by burning one kilogram of coal is 25,000,000 joules (J), how many joules of electrical energy are produced by the combustion of one gram of coal? Answer: 7500 J Standardized Test Prep Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Interpreting Graphics, continued 6.If the input force on this pulley system is 100 N, what is the output force? F.100 N G.200 N H.300 N I.400 N Standardized Test Prep Chapter 12

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Interpreting Graphics, continued 7. How could the amount of force required to raise the bucket be decreased even more? A.Add additional pulleys. B.Increase the length of the rope. C.Thread the rope through the pulleys in opposite order. D.Increase the amount of force on the free end of the rope. Standardized Test Prep Chapter 12