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HOW MACHINES DO WORK? Key Concepts How do machines make work easier? What is a machine’s mechanical advantage? How can you calculate the efficiency of a machine?

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Key Terms Machine Input force Output force Input work Output work Mechanical advantage efficiency

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What is a Machine? A device that allows you to do work in a way that is easier 1. changing the amount of force needed 2. changing the distance over which force is exerted 3. changing the direction in which the force is exerted

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Input and Output Forces Input Force – the force you exert on the machine – makes machine move a certain distance called input distance Output Force – the force the machine exerts over a certain distance called the output distance

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Input and Output Work Input force times input distance = input work Output force times output distance = output work Output work can never be greater than input work

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Changing Force When a machine increases force, you must exert the input force over a greater distance

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Changing Distance When a machine increases distance, you must apply a greater input force

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Changing Direction When a machine changes the direction of the input force, the amount of force and the distance remain the same

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Mechanical Advantage Number of times a machine increases a force exerted on it Mechanical advantage = Output force Input force

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Increasing Force When output force is greater than input force, the mechanical advantage is greater than 1 You exert a force of 10N on a hand held can opener. It exerts an output force of 30 N. MA = Output force = 30N = 3 Input Force 10 N The can opener triples your force

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Increasing Distance For a machine that increases distance the output force is less than the input force and the mechanical advantage is less than one If the input force is 20N and the machine’s output force is 10N MA = Output force = 10N = 0.5 Input force 20N

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Changing Directions For a machine that changes the direction of the force, the input and output forced are the same and the mechanical advantage is 1

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Efficiency of Machines In an ideal situation the work done on machine = the work done by the machine In real situation the output work is always less than the input work

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Friction and Efficiency The less friction there is, the closer the output work is to the input work Efficiency of a machine compares the output work to the input work Efficiency is expressed as a percent

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Calculating Efficiency Efficiency = Output Work X 100% Input Work What is the efficiency of a machine that gives 30 joules of work for every 100 joules put into it? What is the efficiency of a machine that gives 20 joules of work for every 50 joules put into it?

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