work, power, and machines - manasquan public schools · 2009-01-21 · work, power, and machines...
TRANSCRIPT
Work, Power, and Machines
9.1
WorkA quantity that measures the effects of a force acting over a distanceWork = force x distanceW = Fd
Work
Work is measured in:N•mJoules (J)
Work ExampleA crane uses an average force of 5200 N to lift a girder 25 m. How much work does the crane do?
Work ExampleWork = FdWork = (5200 N)(25m)Work = 130000 N • m
= 130000 J
Power
A quantity that measures the rate at which work is donePower = work/timeP = W/t
Power
Watts (W) is the SI unit for power1 W = 1 J/s
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?
Power ExampleWork = FdWork = 19.8 N x 200.0 m= 3960 J
Power = W/tPower = 3960 J/60.0 sPower = 66.0 W
Machines
Help us do work by redistributing the force that we put into themThey do not change the amount of work
Machines
Change the direction of an input force (ex car jack)
Machines
Increase an output force by changing the distance over which the force is applied
(ex ramp)Multiplying forces
Mechanical Advantage
A quantity that measures how much a machine multiples force or distance.
Mechanical Advantage
Output Force
Input Force
Input distanceOutput Distance
Mech. Adv =
Mech. Adv. =
Mech. Adv. example
Calculate the mechanical advantage of a ramp that is 6.0 m long and 1.5 m high.
Mech. Adv. Example
Input = 6.0 mOutput = 1.5 mMech. Adv.=6.0m/1.5mMech. Adv. = 4.0
Simple Machines
9.2
Simple MachinesMost basic machines Made up of two familiesLeversInclined planes
The Lever FamilyAll levers have a rigid arm that turns around a point called the fulcrum.
The Lever FamilyLevers are divided into three classesClasses depend on the location of the fulcrum and the input/output forces.
First Class LeversHave fulcrum in middle of arm.The input/output forces act on opposite endsEx. Hammer, Pliers
First Class LeversOutput Force Input Force
Fulcrum
Second Class LeversFulcrum is at one end.Input force is applied to the other end.Ex. Wheel barrow, hinged doors, nutcracker
Second Class Levers
Output Force
Input Force
Fulcrum
Third Class Levers
Multiply distance rather than force.Ex. Human forearm
Third Class Levers
The muscle contracts a short distance to move the hand a large distance
Third Class Levers
Output distance
Input ForceFulcrum
Pulleys
Act like a modified member of the first-class lever familyUsed to lift objects
Pulleys
Input forceOutput Force
The Inclined Plane
Incline planes multiply and redirect force by changing the distanceEx loading ramp
The Inclined Plane
Turns a small input force into a large output force by spreading the work out over a large distance
A Wedge
Functions like two inclined planes back to back
A Wedge
Turns a single downward force into two forces directed out to the sidesEx. An axe , nail
Not to be mistaken with a wedgIEEEEE
A Screw
Inclined plane wrapped around a cylinder
A Screw
Tightening a screw requires less input force over a greater distance Ex. Jar lids
Compound MachinesA machine that combines two or more simple machinesEx. Scissors, bike gears, car jacks
Energy
9.3-9.4
Energy and WorkEnergy is the ability to do workwhenever work is done, energy is transformed or transferred to another system.
EnergyEnergyEnergy is measured in:Joules (J)
Energy can only be observed when work is being done on an object
Potential Energy PE
the stored energyresulting from the relative positions of objects in a system
PotentialPotential Energy PEEnergy PE
PE of any stretched elastic material is called Elastic PEex. a rubber band, bungee cord, clock spring
Gravitational PEGravitational PE
energy that could potentially do work on an object do to the forces of gravity.
Gravitational PEGravitational PEdepends both on the mass of the object and the distance between them (height)
Gravitational PE Equation
grav. PE= mass x gravity x height
PE = mghor
PE = wh
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?
PE ExamplePE = mghPE=(65kg)(9.8m/s2)(35m)PE = 2.2 x 104 JPE = 22000 J
Kinetic Energythe energy of a moving object due to its motion.depends on an objects mass and speed.
Kinetic EnergyWhat influences energy more: speed or mass?
ex. Car crashesSpeed does
Kinetic Energy Equation
KE=1/2 x mass x speed squared
KE = ½ mv2
KE Example
What is the kinetic energy of a 44 kg cheetah running at 31 m/s?
KE Example
KE = ½ mv2
KE= ½(44kg)(31m/s)2
KE=2.1 x 104 JKE = 21000 J
Mechanical EnergyMechanical Energy
the sum of the KE and the PE of large-scale objects in a system work being done
Nonmechanical Energy
Energy that lies at the level of atoms and does not affect motion on a large scale.
Atoms
Atoms have KE, because they at constantly in motion.KE ↑ particles heat upKE ↓ particles cool down
Chemical Reactions
during reactions stored energy (called chemical energy)is released So PE is converted to KE
Other FormsOther Forms
nuclear fusionnuclear fissionElectricityLight
Energy Transformations
9.4
Conservation of Energy
Energy is neither created nor destroyedEnergy is transferred
Energy Transformation
PE becomes KEcar going down a hill on a roller coaster
Energy Transformation
KE can become PEcar going up a hill KE starts converting to PE
Physics of roller coasters
http://www.funderstanding.com/k12/coaster/