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WORK &MACHINES

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ENDURING UNDERSTANDINGS Work is done when an object moves in the direction of applied forceWork is done when an object moves in the direction of applied force Machines make work easier or more effectiveMachines make work easier or more effective Friction always effects efficiencyFriction always effects efficiency Your body includes multiple levers composed of bones and musclesYour body includes multiple levers composed of bones and muscles

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WHEN IS WORK DONE? For work to be done, an object must move in the direction force is applied.For work to be done, an object must move in the direction force is applied. The amount of work is equal to the applied force times the distance moved:The amount of work is equal to the applied force times the distance moved: Work = force x distance Work = force x distance Work is measured in Joules:Work is measured in Joules: Work = force x distance Joules = Newtons x meters Work = force x distance Joules = Newtons x meters Can you give an example of when you did work today?

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Work or No Work?

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FORCE & WORK

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POWER Power is the rate at which work is done or the amount of work done in a unit of time:Power is the rate at which work is done or the amount of work done in a unit of time: Power = work / time OR Power = force x distance Power = work / time OR Power = force x distance time time Power is measured in watts:Power is measured in watts: Power = work / time Watts = Joules / seconds Power = work / time Watts = Joules / seconds 1 kilowatt = 1000 watts. 1 horsepower = 746 watts.1 kilowatt = 1000 watts. 1 horsepower = 746 watts. How much power is used to move a 20 Newton box a distance of 20 meters in 10 seconds? 40 Watts

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SIMPLE MACHINES A machine makes work easier by changing the force you exert, the distance over which you exert your force, or the direction in which you exert your forceA machine makes work easier by changing the force you exert, the distance over which you exert your force, or the direction in which you exert your force Simple machines include the inclined plane, wedge, screw, lever, pulley, and the wheel and axleSimple machines include the inclined plane, wedge, screw, lever, pulley, and the wheel and axle

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MACHINES & FORCE When you use a machine the input work equals the output workWhen you use a machine the input work equals the output work When a machine increases force, you exert the input force over a greater distance.When a machine increases force, you exert the input force over a greater distance. When a machine increases distance, you apply a greater input force.When a machine increases distance, you apply a greater input force. When a machine changes the direction of the input force, the amount of force and the distance remain the same.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 & EFFICIENCY Mechanical advantage is how many times a machine increases input forceMechanical advantage is how many times a machine increases input force Mechanical advantage equals output force / input forceMechanical advantage equals output force / input force Machine efficiency compares output work to input workMachine efficiency compares output work to input work Machine efficiency = x 100%Machine efficiency = x 100% An ideal machine is one with 100% efficiency. Ideal machines do not exist because of friction; with real machines some work is always lost to friction.An ideal machine is one with 100% efficiency. Ideal machines do not exist because of friction; with real machines some work is always lost to friction. output work input work

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INPUT Input force is the force you exert on the machine. Input force moves the machine a certain distance, called the input distance. Input force times input distance equals input work. You can decrease input force by increasing input distance.

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OUTPUT The force exerted by the machine on an object is the output force.The force exerted by the machine on an object is the output force. The distance the object is moved by the machine is the output distance.The distance the object is moved by the machine is the output distance. The output force times the output distance equals the output work.The output force times the output distance equals the output work.

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Try it... A book weighing 1.5 Newtons is lifted 2 meters. How much work is done? A force of 15 Newtons is used to push a box along the floor a distance of 3 meters. How much work is done? It took 15 Joules to push a chair 5 meters across the floor. With what force was the chair pushed? 1.5 N x 2 m = 3 J 15 N x 3 m = 45 J 15 J / 5 m = 3 N

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A force of 100 Newtons was necessary to lift a rock. A total of 150 Joules of work was done. How far was the rock lifted? It took 500 Newtons of force to push a car 4 meters. How much work was done? A young man exerted a force of 9,000 Newtons on a stalled car but was unable to move it. How much work was done? In each of the preceding problems, can you determine the input force, input distance, output force & output distance? 150 J / 100 N = 1.5 m 500 N x 4 m = 2,000 J 9,000 N x 0 m = 0 J No work done!

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INCLINED PLANES An inclined plane is a flat, sloped surface.An inclined plane is a flat, sloped surface. The ideal mechanical advantage of an inclined plane equals the length of the incline / height of the incline.The ideal mechanical advantage of an inclined plane equals the length of the incline / height of the incline.

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WEDGE A wedge is thick at one end and thin at the other.A wedge is thick at one end and thin at the other. The ideal mechanical advantage of a wedge is the length of the wedge / the width of the wedge.The ideal mechanical advantage of a wedge is the length of the wedge / the width of the wedge.

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SCREWS A screw is an incline plane wrapped around a cylinder.A screw is an incline plane wrapped around a cylinder. The ideal mechanical advantage of a screw is the length of the threads / length of the screw cylinder.The ideal mechanical advantage of a screw is the length of the threads / length of the screw cylinder.

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WHEEL & AXLE A wheel and axle is made of two circular or cylindrical objects fastened together that rotate around a common axis.A wheel and axle is made of two circular or cylindrical objects fastened together that rotate around a common axis. The larger radius is the wheel and the smaller radius is the axle.The larger radius is the wheel and the smaller radius is the axle. The ideal mechanical advantage of the wheel and axle equals the radius of the wheel / radius of the axle.The ideal mechanical advantage of the wheel and axle equals the radius of the wheel / radius of the axle.

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LEVERS A lever is a rigid bar that pivots or rotates on a fixed point.A lever is a rigid bar that pivots or rotates on a fixed point. Ideal mechanical advantage of a lever is the distance from the input force to the fulcrum / distance from the output force to the fulcrum.Ideal mechanical advantage of a lever is the distance from the input force to the fulcrum / distance from the output force to the fulcrum. First class levers change direction of force.First class levers change direction of force. Second class levers increase force but do not change direction.Second class levers increase force but do not change direction. Third class levers increase distance without changing direction.Third class levers increase distance without changing direction.

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TYPES OF LEVERS

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BODY LEVERS

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PULLEYS A pulley is a grooved wheel with a rope or cable wrapped around it.A pulley is a grooved wheel with a rope or cable wrapped around it. A pulley can decrease the input force by increasing input distance, and can also change direction of input force.A pulley can decrease the input force by increasing input distance, and can also change direction of input force. The ideal mechanical advantage of a pulley equals the number of sections of rope that support the object.The ideal mechanical advantage of a pulley equals the number of sections of rope that support the object.

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TYPES OF PULLEYS

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COMPOUND MACHINES A compound machine is a machine that includes two or more simple machines.A compound machine is a machine that includes two or more simple machines. The ideal mechanical advantage of a compound machine is equal to the product of the individual ideal mechanical advantages of the simple machines that make it up.The ideal mechanical advantage of a compound machine is equal to the product of the individual ideal mechanical advantages of the simple machines that make it up.

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