Work, Power and Work, Power and EnergyEnergyChp 10 and 11Chp 10 and 11

Some TermsSome Terms WorkWork

Exerting a force over a distanceExerting a force over a distance EnergyEnergy

The ability to do work (or change the world around you)The ability to do work (or change the world around you) Kinetic energyKinetic energy

Energy due to motionEnergy due to motion Potential energyPotential energy

Energy due to position or compositionEnergy due to position or composition Work-energy theoremWork-energy theorem

work is required to change KEwork is required to change KE

PowerPower The rate at which work is doneThe rate at which work is done

WorkWork

Must be exerted over a distanceMust be exerted over a distance If nothing moves, no work was done on THAT object If nothing moves, no work was done on THAT object

(could still be done on you)(could still be done on you) Need to use the net force to calculate workNeed to use the net force to calculate work

All the vector addition rules still applyAll the vector addition rules still apply SI unit is Joule (J)SI unit is Joule (J) Work is a vector, since force and displacement are Work is a vector, since force and displacement are

vectorsvectors Can be negative, which means the work is being done by Can be negative, which means the work is being done by

the object rather than on itthe object rather than on it

PowerPower

If the same amount of If the same amount of work is done more work is done more quickly, then more power quickly, then more power is exertedis exerted Its harder to do a job Its harder to do a job

quickly than it is to do the quickly than it is to do the same job more slowlysame job more slowly

SI unit is Watts (W)SI unit is Watts (W) Often we use kilowatts Often we use kilowatts

since watts are such a since watts are such a small unitsmall unit

Simple MachinesSimple Machines

A device that makes work A device that makes work easier, either by changing easier, either by changing the distance or direction the distance or direction that a force is exertedthat a force is exerted It doesn’t make the work It doesn’t make the work

lessless If you increase the distance If you increase the distance

a force is exerted, than less a force is exerted, than less force is needed to do the force is needed to do the same amount of worksame amount of work

The machine imparts no The machine imparts no energy of its ownenergy of its own

6 Types of Simple 6 Types of Simple MachinesMachines

Inclined planeInclined plane Ex. rampEx. ramp

LeverLever Ex. crowbarEx. crowbar

Wheel and axelWheel and axel Ex. tireEx. tire

PulleyPulley Ex. Well bucketEx. Well bucket

WedgeWedge Ex. axEx. ax

Screw Screw Ex. Wood screw Ex. Wood screw

Calculating the Benefits Calculating the Benefits of Machinesof Machines

Mechanical advantageMechanical advantage Compares effort and resistance forcesCompares effort and resistance forces

Ideal mechanical advantageIdeal mechanical advantage Compares effort and resistance distancesCompares effort and resistance distances

Efficiency Efficiency Compares how much of the input work gets Compares how much of the input work gets

translated to output worktranslated to output work Often work is lost to heat due to frictionOften work is lost to heat due to friction

Compound MachinesCompound Machines

Result from a combination of 2 or more Result from a combination of 2 or more simple machinessimple machines

The resistance force of one machine The resistance force of one machine becomes the effort force of anotherbecomes the effort force of another Like dominoesLike dominoes

However, the efficiency is less because However, the efficiency is less because friction increases with each additional friction increases with each additional machinemachine

Conservation of EnergyConservation of Energy

Energy can change forms and be passed Energy can change forms and be passed between objects, but it cannot be created between objects, but it cannot be created or destroyedor destroyed

Forms of energy:Forms of energy: KineticKinetic Elastic potential energyElastic potential energy Thermal energyThermal energy Gravitational potential energyGravitational potential energy

CollisionsCollisions

2 types:2 types: ElasticElastic

KE is conserved in the KE is conserved in the collisioncollision

Occurs between hard objectsOccurs between hard objects Inelastic Inelastic

KE is lost in the collision due KE is lost in the collision due to deformation, heat and to deformation, heat and soundsound

Occurs between soft or sticky Occurs between soft or sticky objectsobjects

Energy is still conserved, Energy is still conserved, its just not all kept at KEits just not all kept at KE