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Work, Power, and Machines9.1
Work A quantity that measures the effects of a force acting over a distance Work = force x distance W = Fd
Work Work is measured in: Nm Joules (J)
Work Example A crane uses an average force of 5200 N to lift a girder 25 m. How much work does the crane do?
Work Example Work = Fd Work = (5200 N)(25m) Work = 130000 N m= J
Power A quantity that measures the rate at which work is donePower = work/time P = W/t
Power Watts (W) is the SI unit for power 1 W = 1 J/s
Power Example While rowing in a race, John uses 19.8 N to travel meters in 60.0 s. What is his power output in Watts?
Power Example Work = Fd Power = W/t Power = 3960 J/60.0 sWork = 19.8 N x m= 3960 J Power = W/t Power = 3960 J/60.0 s Power = 66.0 W
Machines Help us do work by redistributing the force that we put into them They do not change the amount of work
Change the direction of an input force (ex car jack)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 Input distance Mech. Adv = Output DistanceOutput Force Mech. Adv. = Input Force
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 m Output = 1.5 m Mech. Adv.=6.0m/1.5m
Simple Machines 9.2
Simple Machines Most basic machines Made up of two families LeversInclined planes
The Lever Family All levers have a rigid arm that turns around a point called the fulcrum.
The Lever Family Levers are divided into three classesClasses depend on the location of the fulcrum and the input/output forces.
First Class Levers Have fulcrum in middle of arm.The input/output forces act on opposite ends Ex. Hammer, Pliers
First Class Levers Input Force Output Force Fulcrum
Second Class Levers Fulcrum is at one end.Input force is applied to the other end. Ex. Wheel barrow, hinged doors, nutcracker
Second Class Levers Output Force Fulcrum Input Force
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 Force Fulcrum
Pulleys Act like a modified member of the first-class lever familyUsed to lift objects
Pulleys Output Force Input force
The Inclined Plane Incline planes multiply and redirect force by changing the distance Ex loading ramp
The Inclined Plane Turns a small input force into a large output force by spreading the work out over a large distance
Functions like two inclined planes back to backA Wedge Functions like two inclined planes back to back
A Wedge Turns a single downward force into two forces directed out to the sides Ex. An axe , nail
Or Wedge Antilles from Star Wars
Not to be mistaken with a wedgIEEEEE
Inclined plane wrapped around a cylinderA Screw Inclined plane wrapped around a cylinder
A Screw Tightening a screw requires less input force over a greater distance Ex. Jar lids
Compound Machines A machine that combines two or more simple machinesEx. Scissors, bike gears, car jacks
Energy and Work Energy is the ability to do workwhenever work is done, energy is transformed or transferred to another system.
Energy Energy is measured in: Joules (J)Energy can only be observed when work is being done on an object
Potential Energy PE the stored energy resulting from the relative positions of objects in a system
Potential Energy PE PE of any stretched elastic material is called Elastic PE ex. a rubber band, bungee cord, clock spring
Gravitational PE energy that could potentially do work on an object do to the forces of gravity.
Gravitational PE depends both on the mass of the object and the distance between them (height)
Gravitational PE Equationgrav. PE= mass x gravity x height PE = mgh or 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 Example PE = mgh PE=(65kg)(9.8m/s2)(35m) PE = 2.2 x 104 JPE = J
Kinetic Energy the energy of a moving object due to its motion.depends on an objects mass and speed.
Kinetic Energy What influences energy more: speed or mass?ex. Car crashes Speed does
Kinetic Energy EquationKE=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 J KE = J
Mechanical 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 up KE particles cool down
Chemical Reactions during reactions stored energy (called chemical energy)is released So PE is converted to KE
Other Forms nuclear fusion nuclear fission Electricity Light
Conservation of EnergyEnergy is neither created nor destroyed Energy is transferred
Energy TransformationPE becomes KE car going down a hill on a roller coaster
Energy TransformationKE can become PE car going up a hill KE starts converting to PE
Physics of roller coasters
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