Work, Power, and Machines
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Work, Power, and Machines
9.1
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WorkA quantity that measures the effects of a force acting over a distance
Work = force x distanceW = Fd
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Work
Work is measured in:NmJoules (J)
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Work ExampleA crane uses an average force of 5200 N to lift a girder 25 m. How much work does the crane do?
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Work ExampleWork = FdWork = (5200 N)(25m)Work = 130000 N m
= 130000 J
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PowerA quantity that measures the rate at which work is done
Power = work/timeP = W/t
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Power
Watts (W) is the SI unit for power
1 W = 1 J/s
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Power ExampleWhile rowing in a race, John uses 19.8 N to travel 200.0 meters in 60.0 s. What is his power output in Watts?
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Power ExampleWork = Fd Work = 19.8 N x 200.0 m= 3960 J
Power = W/tPower = 3960 J/60.0 sPower = 66.0 W
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Machines
Help us do work by redistributing the force that we put into them
They do not change the amount of work
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Machines
Change the direction of an input force (ex car jack)
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Machines
Increase an output force by changing the distance over which the force is applied
(ex ramp)Multiplying forces
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Mechanical Advantage
A quantity that measures how much a machine multiples force or distance.
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Mechanical Advantage
Output Force
Input Force
Input distance
Output DistanceMech. Adv =
Mech. Adv. =
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Mech. Adv. exampleCalculate the mechanical advantage of a ramp that is 6.0 m long and 1.5 m high.
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Mech. Adv. ExampleInput = 6.0 mOutput = 1.5 mMech. Adv.=6.0m/1.5mMech. Adv. = 4.0
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Simple Machines
9.2
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Simple MachinesMost basic machines Made up of two familiesLeversInclined planes
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The Lever FamilyAll levers have a rigid arm that turns around a point called the fulcrum.
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The Lever FamilyLevers are divided into three classes
Classes depend on the location of the fulcrum and the input/output forces.
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First Class LeversHave fulcrum in middle of arm.
The input/output forces act on opposite ends
Ex. Hammer, Pliers
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First Class LeversOutput Force Input Force
Fulcrum
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Second Class LeversFulcrum is at one end.Input force is applied to the other end.
Ex. Wheel barrow, hinged doors, nutcracker
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Second Class Levers
Output Force
Input Force
Fulcrum
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Third Class Levers
Multiply distance rather than force.
Ex. Human forearm
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Third Class Levers
The muscle contracts a short distance to move the hand a large distance
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Third Class Levers
Output distance
Input ForceFulcrum
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PulleysAct like a modified member of the first-class lever family
Used to lift objects
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Pulleys
Input forceOutput Force
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The Inclined Plane
Incline planes multiply and redirect force by changing the distance
Ex loading ramp
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The Inclined Plane
Turns a small input force into a large output force by spreading the work out over a large distance
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A Wedge
Functions like two inclined planes back to back
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A Wedge
Turns a single downward force into two forces directed out to the sides
Ex. An axe , nail
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Or Wedge Antilles from Star Wars
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Not to be mistaken with a wedgIEEEEE
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A Screw
Inclined plane wrapped around a cylinder
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A Screw
Tightening a screw requires less input force over a greater distance
Ex. Jar lids
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Compound Machines
A machine that combines two or more simple machines
Ex. Scissors, bike gears, car jacks
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Energy
9.3-9.4
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Energy and WorkEnergy is the ability to do work
whenever work is done, energy is transformed or transferred to another system.
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EnergyEnergyEnergy is measured in:
Joules (J)Energy can only be observed when work is being done on an object
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Potential Energy PE
the stored energy resulting from the relative positions of objects in a system
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PotentialPotential Energy PEEnergy PEPE of any stretched elastic material is called Elastic PE
ex. a rubber band, bungee cord, clock spring
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Gravitational PEGravitational PEenergy that could potentially do work on an object do to the forces of gravity.
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Gravitational PEGravitational PEdepends both on the mass of the object and the distance between them (height)
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Gravitational PE Equation
grav. PE= mass x gravity x height
PE = mgh or
PE = wh
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PE Example
A 65 kg rock climber ascends a cliff. What is the climber’s gravitational PE at a point 35 m above the base of the cliff?
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PE ExamplePE = mghPE=(65kg)(9.8m/s2)(35m)PE = 2.2 x 104 JPE = 22000 J
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Kinetic Energythe energy of a moving object due to its motion.
depends on an objects mass and speed.
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Kinetic EnergyWhat influences energy more: speed or mass?
ex. Car crashesSpeed does
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Kinetic Energy Equation
KE=1/2 x mass x speed squared
KE = ½ mv2
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KE Example
What is the kinetic energy of a 44 kg cheetah running at 31 m/s?
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KE ExampleKE = ½ mv2
KE= ½(44kg)(31m/s)2
KE=2.1 x 104 JKE = 21000 J
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Mechanical EnergyMechanical Energy
the sum of the KE and the PE of large-scale objects in a system
work being done
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Nonmechanical Energy
Energy that lies at the level of atoms and does not affect motion on a large scale.
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Atoms
Atoms have KE, because they at constantly in motion.
KE particles heat upKE particles cool down
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Chemical Reactions
during reactions stored energy (called chemical energy)is released
So PE is converted to KE
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Other FormsOther Forms
nuclear fusion nuclear fission ElectricityLight
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Energy Transformations
9.4
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Conservation of Energy
Energy is neither created nor destroyed
Energy is transferred
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Energy Transformation
PE becomes KEcar going down a hill on a roller coaster
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Energy Transformation
KE can become PEcar going up a hill KE starts converting to PE
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Physics of roller coasters http://www.funderstanding.com/k12/
coaster/