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Work and machines Making jobs easier

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Work Work is done on an object when a force is exerted on an object that causes the object to move some distance. Work is done on an object when a force is exerted on an object that causes the object to move some distance. No work without motion No work without motion No work without force in the same direction No work without force in the same direction

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Motion and force Person does not work on weights – weights do not move Person does not work on weights – weights do not move Person does work on weights – weights moved Person does work on weights – weights moved Force on weights Summer Olympics 2000 Sydney Australia By Lou Jones http://www.pricespower.com

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Direction and force Horse does not work on the rider – force not in same direction Horse does not work on the rider – force not in same direction Horse does work on the cart - force and motion in the same direction Horse does work on the cart - force and motion in the same direction Force on person motion Force on cart

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Calculating Work Work = Force X Distance Work = Force X Distance Force units = Newton, N Force units = Newton, N Distance units = meter, m Distance units = meter, m Work units = Joule, J Work units = Joule, J J = N*m J = N*m

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The unit of work is the Joule Force x Distance = Work 1 Newton x 1 meter = 1 Newton meter 1 Newton meter = 1 Joule

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Example 1: A high jumper weighs 700 newtons. What work does the jumper perform in jumping over a bar 2.0 meters high? A high jumper weighs 700 newtons. What work does the jumper perform in jumping over a bar 2.0 meters high? Answer: W = F x d Answer: W = F x d W = 700N x 2.0 m W = 700N x 2.0 m = 1400 nm = 1400 nm = 1400 Joules = 1400 Joules

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No work is done if there is no distance! The statue of liberty has been holding up her torch for an awfully long time. How much work has she done? Answer: 0! Although it takes a force to hold the torch against the force of gravity, there is no motion so no work is done.

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Making work easier Machines Machines Change the amount of force you exert Change the amount of force you exert Change the distance over which you exert force Change the distance over which you exert force Change the direction in which you exert force Change the direction in which you exert force Do change the amount of work done Do change the amount of work done

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Total work does not change Work = Force X Distance Work = Force X Distance If a machine allows you to apply a smaller force the distance will increase If a machine allows you to apply a smaller force the distance will increase If a machine reduces the distance the force will increase If a machine reduces the distance the force will increase If a machine allows you to apply a force in a different direction it doesnt change how much force you have to apply or how far you move it If a machine allows you to apply a force in a different direction it doesnt change how much force you have to apply or how far you move it

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Mechanical advantage The number of times a force exerted on a machine is multiplied by the machine Mechanical ad. = Output force Input force If M.A. > 1 machine increases force needed but distance decreases If M.A. < 1 machine reduces force needed but distance increases

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Efficiency Efficiency compares the output work to the input work (%) Efficiency compares the output work to the input work (%) Cannot be greater than 100% Cannot be greater than 100% Efficiency usually < 100% Efficiency usually < 100% Friction Friction

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Types of simple machines Inclined plane Inclined plane Wedge Wedge Screw Screw Lever Lever Wheel and axle Wheel and axle pulley pulley Image taken from: http://www.daniel-wright.district103.k12.il.us/ccheifetz/physics.html

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Inclined plane teacher.scholastic.com/dirtrep/ simple/img/plane.gif

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Wedge teacher.scholastic.com/dirtrep/ simple/img/wedge.gif

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Screw teacher.scholastic.com/dirtrep/ simple/img/screw.gif

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Lever teacher.scholastic.com/dirtrep/ simple/img/lever.gif

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Wheel and axle teacher.scholastic.com/dirtrep/ simple/img/wheel.gif

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Pulley teacher.scholastic.com/dirtrep/ simple/img/pulley.gif

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