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NIS – PHYSICAL SCIENCE

NIS – PHYSICAL SCIENCE. Lecture 19 Work Ozgur Unal. What is Work ?. Work is the energy transferred when a force makes an object move . A force must move an object to do work . Forces do work . Examples : Lifting up a box , pushing a desk forward.

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NIS – PHYSICAL SCIENCE

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  1. NIS – PHYSICAL SCIENCE Lecture 19 Work OzgurUnal

  2. What is Work? Workis theenergytransferredwhen a forcemakes an objectmove. A forcemustmove an objectto do work. Forces do work. Examples: Lifting up a box, pushing a deskforward • If a heavyobjectstays at rest • as youpush it, do you do work? • No, becausetheforceyou • applydoes not do anywork.

  3. WorkandEnergy • Can youstatetherelationshipbetweenworkandenergy? • Whenyou do work on an object (such lifting up an object), do youchangeitsenergy? • Energy is transferredwhen a work is done on an object. • As you lift up an object, its GPE increases. • As youpush an objectforward, youincreaseitskineticenergy. Therefore, you do work.

  4. WorkandEnergy Insomecases, thework done on an objectdecreasestheenergy of thatobject. Example: Slowingdown a fastmovingobjectbyapplying an oppositeforce on it. Therefore, we say that“energy is transferredwhenwork is done”. Energy is transferredfromtheobjectdoingtheworktotheobject on whichwork is done.

  5. CalculatingWork Considerthissituation: Youapply a force on an object at rest tomove it for 20 meters. Doestheamount of energy transferredfromyoutotheobject increaseifyouincreasetheforceyouapply? Doestheamount of energyincreaseifyoumove it withthesameforcefor a longerdistance (i.e. for 40 meters)? • Ifyouapply a biggerforce, theobjectwillgainmorespeed (thereforekineticenergy). • Ifyouapplytheforcefor a longerdistance, theobjectwillgainmorekineticenergy.

  6. CalculatingWork Work done on an object is proportionaltotheforcedoingtheworkandthedistancetheobjecttravels in thedirection of theforce. Work = Force x distance W = F x d What is theunit of work? Is work a vector? • Example:A force of 75 N is exerted on a 45-kg couch, andthecouch moved 5 m. Howmuchwork is done bytheforce in movingthecouch?

  7. When is Work Done? Whenyoucalculatework done on an object, youneedtoconsidertwoquestionsfirst. Doestheobjectmove a distance in thedirection of theforceapplied? Doestheforceact on theobjectthroughoutthewholedistance? • Example: Supposeyouthrow a ballwith a force of 2 N andtheballmoves 10 meters. Can youcalculatethework done? • No, in ordertocalculatework, youneedtofindthedistanceoverwhichtheforce is applied. As theballmoves in theair, you do not applyanyforce on it. Oncetheballleavesyourhand, you do not do anywork on it.

  8. NIS – PHYSICAL SCIENCE Lecture 20 Power OzgurUnal

  9. Power What is power? How is it relatedtowork?

  10. Power Work is theenergytransferredwhen a forcemoves an object. The rate of energy transfer mightdiffer. Example:Pushing an object along a rampslowlyorfast. Example: Let’s say, you do 10 J of work on an objectby lifting it up in 10 seconds. Andyourfriendliftsthesameobject in 20 seconds. • Theratesyou do thesameamount of workaredifferent. • Your rate of doingwork is fasterthanyourfriend.

  11. Power • Poweris theamount of work done in unit time. • Power is the rate at whichwork is done. • Power = Work / Time • P = W / t • Unit of power is Watt (W) Example: Let’s say, you do 10 J of work on an objectby lifting it up in 10 seconds. Andyourfriendliftsthesameobject in 20 seconds. • Theratesyou do thesameamount of workaredifferent. • Your rate of doingwork is fasterthanyourfriend. • Pyou = W / 10 s = 10 J / 10 s = 1 W • Pfriend = W / 20 s = 10 J / 20 s = 0.5 W

  12. Power Example: To lift a babyfrom a crib, 50 J of work is done. Howmuchpower is neededifthebaby is lifted in 0.5 s? • Example: If a runner’spower is 130 W, howmuchwork is done bytherunner in 10 minutes?

  13. PowerandEnergy Rememberthedefinition of work. Power can also be expressed in terms of theenergytransferredfromoneobjecttotheother. Power = Energytransferred / Time P = E / t • Example: Lightbulbs • Thefilament in thelightbulb • convertstheelectricalenergy • intoheatandlight. • Thepower of thelightbulb is • theamount of energytransferred • tothelightbulbeachsecond.

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