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Pearson Prentice Hall Physical Science: Concepts in Action

Pearson Prentice Hall Physical Science: Concepts in Action. Chapter 14 Work, Power and Machines. 14.1 Work and Power. Objectives: 1. Describe the conditions that must exist for a force to do work on an object 2. Calculate the work done on an object 3. Describe and calculate power

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Pearson Prentice Hall Physical Science: Concepts in Action

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  1. Pearson Prentice Hall Physical Science: Concepts in Action Chapter 14 Work, Power and Machines

  2. 14.1 Work and Power • Objectives: • 1. Describe the conditions that must exist for a force to do work on an object • 2. Calculate the work done on an object • 3. Describe and calculate power • 4. Compare units of watts and horsepower as they relate to power

  3. Conditions for Work • Def: work is the product of force times distance • For a force to do work on an object, some of the force must act in the same direction as the object moves • If the object does not move, no work is done • Work depends on direction • Any part of a force that does not act in the direction of motion does no work on the object

  4. Calculating Work • Work = Force x Distance • The units for force are Newtons, N • Recall from chapter 12 that 1 N = 1 kg*m/s2 • The unit for distance is the meter, m • The unit for force is 1 N*m or 1 kg*m2/s2 which equals one joule, abbreviated J

  5. Calculate Power • Def: power is the rate of doing work • Doing work at a faster rate requires more power • To increase power, increase the amount of work done in a given time OR do a given amount of work in less time • Power = Work/Time • The unit of work is joules (J) • The unit of time is seconds (s) • J/s = watts (W) & the unit of power is watts

  6. Watts and Horsepower • One horsepower equals 746 watts • James Watt defined horsepower as the power output of a very strong horse • Watt did not want to exaggerate the power of steam engines

  7. 14.2 Work and Machines • Objectives: • 1. Describe what a machine is and how it makes work easier to do • 2. Relate work input of a machine to work output of the machine

  8. What a Machine is & How it Makes Work Easier • Def: a machine is a device that changes a force • Machines make work easier to do • Machines change the size of a force needed, the direction of the force, or the distance over which a force acts • Some machines increase distance over which to exert a force, decreasing the amount of force needed • Some machines exert a large force over a short distance • Some machines change the direction of the applied force

  9. Work Input and Work Output • Because of friction, the work done BY a machine is always less than the work done ON a machine • Def: work input is work done by the input force acting through input distance • Def: work output is force exerted by a machine • Def: output distance is the distance of the output force

  10. 14.3 Mechanical Advantage and Energy • Objectives: • 1. Compare a machine’s actual mechanical advantage to it ideal mechanical advantage • 2. Calculate the ideal and actual mechanical advantages of various machines • 3. Explain why efficiency of a machine is always less than 100% • 4. Calculate a machine’s efficiency

  11. Actual and Ideal Mechanical Advantage + Calculations • Def: mechanical advantage is the number of times that a machine increases an input force • Actual MA = output force/input force • Def: ideal mechanical advantage is the MA in the absence of friction • Friction is always present, so the actual MA of a machine is always less than the ideal MA • Ideal MA= input distance/output distance • There are no units with MA

  12. Efficiency Calculation & Why it is Less Than 100% • Def: efficiency of a machine is the percentage of work input that becomes work output • Efficiency is always less than 100% since friction is always present • Efficiency = work output/work input x 100%

  13. 14.4 Simple Machines • Objectives: • 1. Describe the six types of simple machines • 2. Explain what determines the mechanical advantage of the six types of simple machines

  14. Six Types of Simple Machines & MA • The six types of simple machines are the lever, wheel and axle, inclined plane, wedge, screw and pulley • Def: a lever is a rigid bar free to move about a fixed point • Def: a fulcrum is the fixed point a lever moves around • Def: the input arm is the distance between the input force and the fulcrum

  15. Def: the output arm is the distance between the output force and the fulcrum • For a lever: MA = input arm/output arm • There are 3 classes of levers: first, second and third class • For first class levers the fulcrum is located between the input force and the output force • MA for first class levers is =, < or > 1 • Examples: seesaws, scissors, tongs, screwdriver

  16. For second class levers, the output force is located between the input force and fulcrum • MA is always >1 for second class levers • Example: wheelbarrow • For third class levers, the input force is located between the fulcrum and output force • MA is always <1 for third class levers • Examples: baseball bats, hockey sticks, golf clubs & brooms

  17. Def: a wheel and axle consists of 2 disks or cylinders, each one with a different radius • Example: steering wheel • To calculate MA for wheel and axle, divide the radius (or diameter) where the input force is exerted by the radius (or diameter) where the output force is exerted • Def: an inclined plane is a slanted surface along which a surface moves an object to a different elevation • Example: ramp in front of buildings • The ideal MA for an inclined plane is the distance along the plane divided by its height

  18. Def: a wedge is V-shaped object whose sides are two inclined planes sloped toward each other • Example: flat head screwdriver • A thin wedge of given length has a greater ideal MA than a thick wedge of the same length • Def: a screw is an inclined plane wrapped around a cylinder • Screws with threads closer together have a greater ideal MA

  19. Def: a pulley consists of a rope that fits into a groove in a wheel • The MA of a pulley or pulley system is equal to the number of rope sections supporting the load being lifted • Def: a fixed pulley is a wheel attached in a fixed location • The ideal MA of a fixed pulley is always 1 • Def: a movable pulley us attached to the object being moved • The ideal MA of a movable pulley is 2

  20. Def: a pulley system is a combination of fixed and movable pulleys that operate together • MA depends on pulley arrangement • Def: a compound machine is a combination of two or more simple machines that operate together • Examples: cars, washing machines, clocks

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