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CHAPTER 5 WORK AND MACHINES

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WORK The transfer of energy to cause or make an object move

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WORK If there is no movement, no work is done Ex: Lifting a text book (work) vs pushing on a brick wall (no movement = no work)

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WORK AND ENERGY When work is done, a transfer of energy occurs You become tired when you walk or carry things up a flight of stairs

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JOULE The unit used to express work (J)

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HOW DO YOU CALCULATE WORK? Work = Force X Distance Or W = F x D

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TRY IT You apply a force of 10 N to a shopping cart. You moved the cart 10 m. How much work did you do on the shopping cart?

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POWER The rate at which work is done or energy is transferred

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WATT The unit used to express power. Symbol for Watt (W) Usually written in italics

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HOW DO YOU CALCULATE POWER? Power = Work/Time Or P = W/T

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TRY IT It takes you 5 s to do 100 J of work on a shopping cart to move it down the sidewalk. What is your power output?

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MACHINES A device that makes doing work easier Changes force (increases it) DOES NOT change the amount of work

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WORK DONE BY MACHINES Input force – force that is applied to the machine = F in Output force – the force applied by the machine = F out

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WORK INPUT B/c of friction machines aren‘t 100% efficient Work Input= force YOU exert on machine = W in Input Distance = distance YOU are using Ex: Rowing boat Input force - how hard you pull oars Input distance - how much oar handles move

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WORK OUTPUT Output Force = force exerted by machine Output distance = distance machine uses Work Output = what machine does/accomplishes = Wout Ex: Rowing boat Output force - how hard oars push water Output distance - distance oar ends move in water Work Output - moving through water

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WORK INPUT AND OUTPUT

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CONSERVING ENERGY When energy is used by a machine, some of the energy is transferred as heat due to friction W out is never greater than W in W out is always smaller than W in

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MECHANICAL ADVANTAGE Advantage of using a particular machine Mechanical Advantage Equation: Mechanical Advantage = output force (in newtons) input force (in newtons) OR MA = F out / F in

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TRY IT Calculate the mechanical advantage of a hammer if the input force is 125N and the output force is 2000N.

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MECHANICAL EFFICIENCY Measure of how much of the work put into a machine is changed into useful work output by the machine Calculating Efficiency: Efficiency(%) = output work (in joules) x 100% input work (in joules OR efficiency = W out / W in x 100% Higher the number, the more efficient

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TRY IT Find the efficiency of a machine that does 800J of work if the input work is 2400J.

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TYPES OF MACHINES (SIMPLE MACHINES) 6 Simple Machines make other machines 1.Lever Family 1.Lever 2.Pulley 3.Wheel & axle 2.Inclined Plane Family 1.Simple inclined plane 2.Wedge 3.Screw

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LEVERS Have a rigid arm that turns around a fulcrum 3 classes: 1.1st class 2.2nd class 3.3rd class

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1ST CLASS LEVER Fulcrum in middle of arm Exs: scissors, pliers, hammer claw, seesaw

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2ND CLASS LEVER Fulcrum at one end and force at other end Load in middle Exs: wheelbarrow, door, nutcracker (MA > 1)

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3RD CLASS LEVER Fulcrum at one end, force in middle, and load on other end Exs: tweezers, biceps

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PULLEY Fulcrum in middle of circle Lever = rope More pulleys – easier work

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TYPES: A.) Fixed wheel attached in a fixed position MA = 1

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B.) Movable attached to the object being moved MA = 2

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C.) Block and Tackle Combination of fixed and movable pulleys MA = depends on the number of rope segments

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WHEEL & AXLE 2 different sized wheels Axle is fulcrum, wheel is lever Exs: steering wheel, screw driver Gears – toothed W & A

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MECH. ADV. OF A WHEEL AND AXLE MA = Radius of Wheel/Radius of Axle

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INCLINED PLANE Spreads work over long distances Easier to use a long ramp Exs: stairs, ramps, escalators

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MECH. ADV. OF INCLINE PLANES MA = Length/Height (L/H)

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WEDGE 2 inclined planes back to back Holds together or separates objects Exs: nails, axes

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WEDGES

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SCREW Threads are spiraled incline plane Exs: jar lid, spiral staircase

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SCREW

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COMPOUND MACHINE More than 1 simple machine together Ex: scissors - lever (handles) and wedge (blade)

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Chapter 5 Work and Machines

Chapter 5 Work and Machines

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