Whatisalwayspresentbutnevervisible?ENERGY!Energyisadifficultconcepttounderstandbecauseitisnotaconcreteobjectthatyoucanseeortouch.Tounderstandwhatenergyis,youmustfirstunderstandwhatitdoes.Thatis,althoughenergyisn’tvisible,youcandetectevidenceofenergy.Jumping,movingawheelchair,eating,andsingingallrequireenergy.Nonlivingthingsalsouseenergy—aclock,vacuumcleaner,andmechanicaltoysallrequireenergytomove.Workisinvolvedwheneveranythingmovesadistance,andenergyisneededtodowork.Therefore,energyisdefinedastheabilitytodowork.Muchlikemassorvolume,energyisapropertyofanobject.It’sjustthatenergyismoreabstractthansomeotherproperties.Althoughenergyitselfisn’tvisible,youcandetectevidenceofenergy.Movement,sound,heat,andlightprovideevidencethatenergyispresentandbeingused.Soundisproducedwhenwestrikesomething.Butdoessounddowork?Yes,soundcanmovethings.Soundwavesmovethetinybonesinyourearsandshakewindowswhenaloudtruckpassesby.Soundwavesarealsoevidentinthevibrationsfromplayingaradio.Ourbodyisworkingevenwhenitappearstobestill.Breathing,blinking,anddigestingfoodallrequireenergy.Forustodotheseactivities,ourbodiesburntheenergyinfood.Weknowthisishappeningbecausewefeelwarm(burninggeneratesthermalenergy,orheat).Therefore,heatisevidencethatenergyisbeingused.Ifenergyistheabilitytodowork,howdoesthermalenergyfitintothisdefinition?Thermalenergycanmeltanicecubeormakewaterboil.Therefore,thedefinitionofenergycanbeamendedtoenergyistheabilitytodoworkortoorganizeorchangematter.Lightisanotherobservableformofenergy.Lightcanchangethings.Whenlightshinesonyourarm,itmakesitfeelwarm.Whenlightshinesonagreenplant,theplantcanmakefood.Sinceenergyisapropertyofmatter,scientistshavediscoveredwaystomeasureandquantifyenergy.Measuringenergyhelpstounderstandhowitisused,howitchangesforms,andhowtoincreaseenergyefficiency.VariousFormsofPotentialEnergyGravitationalPotentialEnergyWhensomethingisliftedorsuspendedinair,workisdoneontheobjectagainstthepullofgravity.Thisworkisconvertedtoaformofpotentialenergycalledgravitationalpo