work and machines work – energy transferred when a force makes an object move 2 conditions must...
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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 must be in same direction as
applied force No work is done to something you carry
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How 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/time
▫Work and Energy are the same thing!!!
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Work problems• Chase 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?
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Power problems• Danielle 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?
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Machines
•Devices that make work easier.• But how???
▫They increase the force applied to an object▫They increase the distance over which the
force is applied
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Ideal Machines (or Perfect Machines)
•Machines involved 2 types of work:▫Work input – what YOU put into the
machine▫Work output – what you get out of the
machine•Win = Wout
▫This is NOT possible…why? Friction!!!!
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Mechanical advantage
•The ratio of output force to input force•MA = output force (N)/input force (N)
▫MA = Fout/Fin
•Ideal Mechanical Advantage (IMA) is the MA without friction
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Efficiency
•(Work output/Work input) X 100•How can efficiency be increased?
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Simple Machines
• Have few or no moving parts
• Make work easier• Can be combined to
create complex machines• Six simple machines:
Lever, Inclined Plane, Wheel and Axle, Screw, Wedge, Pulley
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Lever• 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 force
• Trade off: must move lever large distance to move load small distance
• There are 3 types of levers
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1st Class Lever
•The fulcrum is located between the effort and the load
•Direction of force always changes
•Examples are scissors, pliers, and crowbars
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2nd Class Lever
•The resistance is located between the fulcrum and the effort
•Direction of force does not change
•Examples include bottle openers and wheelbarrows
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3rd Class Lever•The effort is
located between the fulcrum and the resistance
•Direction of force does not change, but a gain in speed always happens
•Examples include ice tongs, tweezers and shovels
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Mechanical 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 load
• For our example, MA = 10/5 = 2
• Distance from effort to fulcrum: 10 feet
• Distance from load to fulcrum: 5 feet
MA =Distance, effort - fulcrumDistance, load - fulcrum
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Inclined 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 distance
• Trade off: the distance the load must be moved would be greater than simply lifting it straight up
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Mechanical 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 slope
• So for our plane MA = 15 feet/3
feet = 5
• Let’s say S = 15 feet, H = 3 feet
MA =Length of SlopeHeight of Plane
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Wheel and Axle•A larger circular wheel
affixed to a smaller rigid rod at its center
•Used 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
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Mechanical 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 axle
• So for our wheel and axle MA = 10”/2” = 5
2"
10"
MA =Radius of WheelRadius of Axle
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Screw
•An inclined plane wrapped around a rod or cylinder
•Used to lift materials or bind things together
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Mechanical Advantage: Screw• The Mechanical
advantage of a screw is the circumference of the screwdriver divided by the pitch of the screw
• The pitch of the screw is the number of threads per inch
• So for our screwdriver
MA = 3.14”/0.1” = 31.4
Diam.=1"
10 threadsper inch
Circumference = ∏ x 1” = 3.14”
Pitch = 1/10” = 0.1”
MA = Circumference of ScrewdriverPitch of Screw
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Wedge
•An inclined plane on its side
•Used to cut or force material apart
•Often used to split lumber, hold cars in place, or hold materials together (nails)
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Mechanical 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 end
• So for our wedge, MA = 6”/2” = 3• They are one of the
least efficient simple machines
2"
6"
MA = Length of SlopeThickness of Widest End
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Pulley• A rope or chain free to turn
around a suspended wheel• By pulling down on the rope,
a load can be lifted with less force
• Trade 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 rope
• IMA = input arm length/ output arm length
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The trick is WORK
•Simple machines change the amount of force needed, but they do not change the amount of work done
•What is work?▫Work equals force times distance▫W = F x d
•By increasing the distance, you can decrease the force and still do the same amount of work