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 4. Compare units of watts and horsepower as they relate to power

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

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

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

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

1. Describe what a machine is and how it makes work easier to do 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

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

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

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

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

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%

1. Describe the six types of 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

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

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

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

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

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

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

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