Presentation on theme: "Work and Machines Work – energy transferred when a force makes an object move 2 conditions must apply for there to be work: The object must move Movement."— Presentation transcript:
1Work and MachinesWork – energy transferred when a force makes an object move2 conditions must apply for there to be work:The object must moveMovement must be in same direction as applied forceNo work is done to something you carry
2How do you calculate work? Work (Joules) = force (N) x distance (m)Power is the amount of work done in one second.Power (Watts) = work (J)/time (sec)What is a Kilowatt?Also…Power = Energy/timeWork and Energy are the same thing!!!
3Work problemsChase lifts a 100 kg (220 lbs.) barbell 2 meters. How much work did he do?Caitlin pushes and pushes on a loaded shopping cart for 2 hours with 100 N of force. The shopping cart does not move. How much work did Caitlin do?
4Power problemsDanielle exerts 40 N of force to move an object 2 m in 4 seconds. What was her power?Charles bench presses 100 kg (220 lbs) 0.5 m for 20 reps in 20 seconds. What was the power of this impressive man of steel?
5Machines Devices that make work easier. But how??? They increase the force applied to an objectThey increase the distance over which the force is applied
6Ideal Machines (or Perfect Machines) Machines involved 2 types of work:Work input – what YOU put into the machineWork output – what you get out of the machineWin = WoutThis is NOT possible…why?Friction!!!!
7Mechanical advantage The ratio of output force to input force MA = output force (N)/input force (N)MA = Fout/FinIdeal Mechanical Advantage (IMA) is the MA without friction
8Efficiency (Work output/Work input) X 100 How can efficiency be increased?
9Simple Machines Have few or no moving parts Make work easier Can be combined to create complex machinesSix simple machines: Lever, Inclined Plane, Wheel and Axle, Screw, Wedge, Pulley
10Lever A rigid board or rod combined with a fulcrum and effort By varying position of load and fulcrum, load can be lifted or moved with less forceTrade off: must move lever large distance to move load small distanceThere are 3 types of levers
111st Class Lever The fulcrum is located between the effort and the load Direction of force always changesExamples are scissors, pliers, and crowbars
122nd Class LeverThe resistance is located between the fulcrum and the effortDirection of force does not changeExamples include bottle openers and wheelbarrows
133rd Class LeverThe effort is located between the fulcrum and the resistanceDirection of force does not change, but a gain in speed always happensExamples include ice tongs, tweezers and shovels
14Mechanical Advantage: Lever The mechanical advantage of a lever is the distance from the effort to the fulcrum divided by the distance from the fulcrum to the loadFor our example,MA = 10/5 = 2Distance from effort to fulcrum: feetDistance from load to fulcrum: 5 feet
15Inclined Planes A slope or ramp that goes from a lower to higher level Makes work easier by taking less force to lift something a certain distanceTrade off: the distance the load must be moved would be greater than simply lifting it straight up
16Mechanical Advantage: Inclined Plane The mechanical advantage of an inclined plane is the length of the slope divided by the height of the plane, if effort is applied parallel to the slopeSo for our planeMA = 15 feet/3 feet = 5Let’s say S = 15 feet, H = 3 feet
17Wheel and AxleA larger circular wheel affixed to a smaller rigid rod at its centerUsed to translate force across horizontal distances (wheels on a wagon) or to make rotations easier (a doorknob)Trade off: the wheel must be rotated through a greater distance than the axle
18Mechanical Advantage: Wheel and Axle The mechanical advantage of a wheel and axle system is the radius of the wheel divided by the radius of the axleSo for our wheel and axle MA = 10”/2” = 5
19Screw An inclined plane wrapped around a rod or cylinder Used to lift materials or bind things together
20Mechanical Advantage: Screw The Mechanical advantage of a screw is the circumference of the screwdriver divided by the pitch of the screwThe pitch of the screw is the number of threads per inchSo for our screwdriverMA = 3.14”/0.1” = 31.4Circumference = ∏ x 1” = 3.14”Pitch = 1/10” = 0.1”
21Wedge An inclined plane on its side Used to cut or force material apartOften used to split lumber, hold cars in place, or hold materials together (nails)
22Mechanical Advantage: Wedge Much like the inclined plane, the mechanical advantage of a wedge is the length of the slope divided by the width of the widest endSo for our wedge,MA = 6”/2” = 3They are one of the least efficient simple machines
23Pulley A rope or chain free to turn around a suspended wheel By pulling down on the rope, a load can be lifted with less forceTrade off: no real trade off here; the secret is that the pulley lets you work with gravity so you add the force of your own weight to the ropeIMA = input arm length/ output arm length
24The trick is WORKSimple machines change the amount of force needed, but they do not change the amount of work doneWhat is work?Work equals force times distanceW = F x dBy increasing the distance, you can decrease the force and still do the same amount of work