Presentation on theme: "Chapter 12 – Work and Machines"— Presentation transcript:
1 Chapter 12 – Work and Machines Work is when a force is used to make something move.Work = Force (Distance)Work unitsEnglish units will be foot.poundsMetric units will be newton.meters (which is called a joule)
2 Example problem:Use the four step method to determine how much work is done if a 50 newton force is used to drag a rock 30 meters.The force used in this equation is the force in the direction of motion.40 newtons Force30 newtons Force30 newtons Force used to move box
3 MachinesA machine is a device with which you can do work in a way that is easier or more effectiveA machine can be simple or complexA machine makes work easier by:Changing the amount of force you exert orChanging the distance over which you exert the forceThe direction in which you exert your force
4 Input force – force you exert on the machine Output force – force exerted by the machineOutput ForceMechanical advantage =Input ForceMultiplying Force (mechanical advantage greater than 1)If the input force is less than the output force, the input force must be exerted over a greater distance.Examples: a car jack, pulling nails with a hammer
5 Multiplying distance (mechanical advantage less than 1) If the input force is greater than the output force, the output end of the machine will move a greater distance.Examples: a fan, a hockey stick, high gear on a bicycle or carChanging Direction of ForceExample: a pulley, a lever
6 EfficiencyEfficiency is the total work or energy input into a machine compared to the useful work or energy output of the machineOutput WorkEfficiency = x 100Input Work
7 If there was no energy loss through friction or other causes, the efficiency of a machine would be 100%No machine is 100% efficient, although some simple machines are very close to 100% efficient
8 For simple machines Input work ≈ Output work Lift a distance of 2 ft. Push down a distance of 5 ft.20 lbs50 lbsInput work = F (D)Input work = 20 lb (5 ft.)Input work = 100 ft-lbsOutput work = F (D)Output work = 50 lb (2 ft.)Output work = 100 ft-lbs100 ft-lbs ≈ 100 ft-lbs
9 Use the efficiency equation to determine the approximate efficiency of the lever system. output workEfficiency = x 100 =input work100 ft-lbsx efficiency = 100%
10 Example efficiency problems: Use the four step method to solve these problems Determine the efficiency of a machine that requires a work input of 224 foot pounds for a work output of 200 foot pounds.A hydraulic jack requires thirty ½ foot down ward strokes of 20 pounds each to lift an 1150 pound object a distance of .25 feet. Calculate the efficiency of the jack.
11 Ideal Mechanical Advantage – the mechanical advantage of a machine if there was no friction. This can be calculated.Actual Mechanical Advantage – the real mechanical advantage of a machine. This can be different for each machine and must be measured.
12 Part 2 – Simple Machines The 6 types of simple machines are: The inclined planeThe wedgeThe screwThe leverThe wheel and axleThe pulley
14 The Inclined Plane Mechanical advantage (MA)= output force/input force Ideal MA = length of the incline/height of inclineIf there was no friction the actual MA would equal ideal MA. In real life it is always less.Incline lengthIncline height
22 Levers (3 types) First Class Lever (a pry or teeter-totter) Second class Lever (wheel barrel)Third class LeverOutput forceInput forceInput forceOutput forceInput forceOutput forceDistance from fulcrum to input forceIdeal MA =Distance from fulcrum to output force
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