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Chapter 4.2 How Machines Do Work?

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- How Machines Do Work Input and Output Work The amount of input work done by the gardener equals the amount of output work done by the shovel.

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Input & Output work Input force = the FORCE you exert on the machine. Output force = the FORCE the machine exerts ON THE OBJECT

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Calculating Input & Output work Input work Input force x Input distance Output work Output force x Output distance When you use a machine, the amount of output work can never be greater than the amount of input work.

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- How Machines Do Work What Is a Machine? A machine makes work easier by changing at least one of three factors. A machine may change the amount of force you exert, the distance over which you exert your force, or the direction in which you exert your force.

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Mechanical Advantage If you compare the input force to the output force, you can find the advantage of using a machine. A machine’s mechanical advantage is the number of times a machine increases a force exerted on it.

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Calculating Mechanical advantage Mechanical advantage = Output force input force Examples: A machine that increases force: output force is greater than input force. ex. a can opener

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For a machine that increases distance the output force is less than the input force. Ex. A wooden spoon vs. a regular spoon. For a machine that changes direction the input force will equal the output force Ex. A weight pulley machine The mechanical advantage of these types of machines will always be 1.

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Mechanical Advantage 400 N Interpreting Data: If an 80-N input force is exerted on Ramp 2, what is the output force? - How Machines Do Work

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Efficiency of Machines So far you learned that input work is equal to output work. This does not happen is real life. WHY??? FRICTION In every machine, some work is wasted overcoming friction. Force must be applied to overcome the friction of the machine.

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Efficiency of machines The efficiency of a machine compares the output work to the input work. Efficiency is calculated as a percentage (%) Efficiency = output work input work X 100

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Calculating Efficiency You do 250,000 J of work to cut a lawn with a hand mower. If the work done by the mower is 200,000 J, what is the efficiency of the lawn mower? Read and Understand What information have you been given? Input Work (W input ) = 250,000 J Output Work (W output ) = 200,000 J - How Machines Do Work

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Calculating Efficiency You do 250,000 J of work to cut a lawn with a hand mower. If the work done by the mower is 200,000 J, what is the efficiency of the lawn mower? Plan and Solve What quantity are you trying to calculate? The efficiency of the lawn mower = __ What formula contains the given quantities and the unknown quantity? Efficiency = Output work/Input work X 100% Perform the calculation. Efficiency = 200,000 J/250,000 J X 100% Efficiency = 0.8 X 100% = 80% The efficiency of the lawn mower is 80 percent.

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Real & Ideal Machines Real machines Efficiency is always less than 100% Ideal machines Efficiency is always 100% ( these machines do not exist)

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