### Similar presentations

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Work and Power What Is Work? Work is the transfer of energy to an object by using a force that causes the object to move in the direction of the force. Transfer of Energy One way you can tell that work is being done is that energy is transferred. Chapter 8

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Work and Power What Is Work?, continued Difference Between Force and Work Applying a force doesn’t always result in work being done. Force and Motion in the Same Direction For work to be done on an object, the object must move in the same direction as the force. Chapter 8

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Work and Power How Much Work? Same Work, Different Force Work depends on distance as well as force. Chapter 8

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Work and Power How Much Work?, continued Calculating Work The amount of work (W) done in moving an object can be calculated by multiplying the force (F) applied to the object by the distance (d) through which the force is applied: Chapter 8 The unit used to express work is the newton-meter (N  m), which is more simply called the joule. W  F  d

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Work and Power Power: How Fast Work Is Done Calculating Power Power is the rate at which energy is transferred. To calculate power (P), you divide the amount of work done (W) by the time (t) it takes to do that work: Chapter 8 The unit used to express power is joules per second (J/s), also called the watt. One watt (W) is equal to 1 J/s.  P  W t

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 Work and Power Power: How Fast Work Is Done, continued Chapter 8 Increasing Power It may take you longer to sand a wooden shelf by hand than by using an electric sander, but the amount of energy needed is the same either way. Only the power output is lower when you sand the shelf by hand.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 What Is a Machine? Machines: Making Work Easier A machine is a device that makes work easier by changing the size or direction of a force. Chapter 8

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 What Is a Machine? Machines: Making Work Easier, continued Work In, Work Out The work that you do on a machine is called work input. The work done by the machine on an object is called work output. How Machines Help Machines allow force to be applied over a greater distance, which means that less force will be needed for the same amount of work. Chapter 8

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 What Is a Machine? Machines: Making Work Easier, continued Same Work, Different Force Machines make work easier by changing the size or direction of the input force. The Force-Distance Trade Off When a machine changes the size of the force, the distance through which the force is exerted must also change. Chapter 8

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 What Is a Machine? Mechanical Efficiency The less work a machine has to do to overcome friction, the more efficient the machine is. Mechanical efficiency is a comparison of a machine’s work output with the work input. Calculating Efficiency A machine’s mechanical efficiency is calculated using the following equation: Chapter 8 mechanical efficiency  work output work input  100

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 What Is a Machine? Mechanical Efficiency, continued Perfect Efficiency? An ideal machine would be a machine that had 100% mechanical efficiency. Ideal machines are impossible to build, because every machine has moving parts. Moving parts always use some of the work input to overcome friction. Chapter 8

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 Types of Machines Levers A lever is a simple machine that has a bar that pivots at a fixed point, called a fulcrum. First-Class Levers With a first-class lever, the fulcrum is between the input force and the load. Chapter 8

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 Types of Machines Pulleys A pulley is a simple machine that consists of a wheel over which a rope, chain, or wire passes. Fixed Pulleys A fixed pulley is attached to something that does not move. Chapter 8

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 Types of Machines Pulleys, continued Movable Pulleys Unlike fixed pulleys, movable pulleys are attached to the object being moved. Blocks and Tackles When a fixed pulley and a movable pulley are used together, the pulley system is called a block and tackle. Chapter 8

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 Types of Machines Wheel and Axle What Is a Wheel and Axle? A wheel and axle is a simple machine consisting of two circular objects of different sizes. Chapter 8

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 Types of Machines Wheel and Axle, continued Mechanical Advantage of a Wheel and Axle The mechanical advantage of a wheel and axle can be found by dividing the radius (the distance from the center to the edge) of the wheel by the radius of the axle. Chapter 8

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 Types of Machines Inclined Planes An inclined plane is a simple machine that is a straight, slanted surface. Mechanical Advantage of an Inclined Plane The mechanical advantage (MA) of an inclined plane can be calculated by dividing the length of the inclined plane by the height to which the load is lifted. Chapter 8

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 Types of Machines Inclined Planes, continued Wedges A wedge is a pair of inclined planes that move. Mechanical Advantage of Wedges can be found by dividing the length of the wedge by its greatest thickness. Chapter 8

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 Types of Machines Inclined Planes, continued Screws A screw is an inclined plane that is wrapped in a spiral around a cylinder. Mechanical Advantage of Screws The longer the spiral on a screw is and the closer together the threads are, the greater the screw’s mechanical advantage is. Chapter 8

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 3 Types of Machines Compound Machines What Are Compound Machines? Compound machines are machines that are made of two or more simple machines. Mechanical Efficiency of Compound Machines The mechanical efficiency of most compound machines is low, because compound machines have more moving parts than simple machines do. Thus, there is more friction to overcome. Chapter 8