Work,Power,andMechanicalAdvantageReview

LG1:SWBATdefineworkandcalculatetheworkdoneonanobject.

€

Work = Force × distanceJoules!

Amountofenergyusedbypersonor

machine

Howmuchforceisappliedtothe

object

Thedistanceforwhichtheforceisappliedtothe

object

W F d

Newtons! meters!

Example1

Krystalli;edaboxwithaforceof50N.Sheli;edthebox2m.Howmuchworkdidshedo?

Answer

Force=50Ndistance=2m

Work=ForcexdistanceW=50Nx2mW=100Joules(100J)

Krystaldid100Joulesofwork.

Example2

Arockclimberweighs600N.(TheforceofgravityacKngonhimis600Ndownward.)Tomakehimselfmoveuponarock,heneedstopushupwith600Nofforcetoovercometheforceofgravity.Ifhepusheshimself10muptherock,howmuchworkhashedone?

600N

600N

10m

Answer

Force=600N

distance=10m

Work=Forcexdistance

W=600Nx10mW=6000Joules(6000J)

Theclimberdoes6000Joulesofworkduringthisclimb.

600N

600N

10m

CheckForUnderstanding

Whichofthefollowingisthebestexampleofworkbeingdoneonanobject?

A.  Holdinga50kgbarbellB.  LiSingabagofgroceriesC.  KeepingaboardinplaceD.  Pushingacarthatwillnotmove

LG2:SWBATdefinepowerandcalculatethepowerapersonor

machineuses.

€

Power = Worktime

Howquicklyworkisdone

Howmuchworkisdoneontheobject

Howlongittakesfortheworktobe

done

seconds!

Joules!

Watts!

€

P = Wt

Example1

Starlandoes500Jofworkli;ingaheavyboxintoatruck.Ittakeshim10secondstoli;thebox.HowmuchpowerdidStarlanuse?

Answer

Work=500JKme=10s

Starlanused50waVsofpowerliSingthebox.

€

P = Wt

=500 J10 s

= 50 W

Example2

KendrickliSsa1000Nweightaheightof1min2seconds.TimberlandliSsthesame1000Nweightaheightof1min4seconds.

(a) HowmuchworkdoesKendrickdo?(b) HowmuchpowerdoesKendrickuse?(c) HowmuchworkdoesTimberlanddo?(d) HowmuchpowerdoesTimberlanduse?

Answer

(a) W=Fd=1000Nx1m=1000J

(b) 

(c) W=Fd=1000Nx1m=1000J

(d) 

€

P = Wt

=1000 J

2 s= 500 W

€

P = Wt

=1000 J

4 s= 250 W

Example3

Alightbulbhasapowerof80waSs.Ifthelightbulbisonfor40seconds,howmuchworkwillthelightbulbdo?(Howmuchenergywillituse?)

80waSs

40seconds

Answer

Power=80WKme=40s

Thelightbulbwoulddo3200Jofwork(release3200Jofenergy)in40seconds.€

P = Wt

80W = W40s

W = (80W) × (40s) = 3200 J

80waSs

40seconds

ChallengeQues[on

Howmuchpowerisrequiredtoli;a30NboxaverKcaldistanceof20mifthisworkisaccomplishedin40seconds?

CheckForUnderstandingAcraneliSstwo100kgloadstothetopofabuilding.ThefirstloadisliSedin10secondsandthesecondloadisliSedin15seconds.Whichstatementbelowistrue?

A.  ThecranedoesmoreworkandexpendsmorepowerforthefirstliS.

B.  ThecranedoesthesameworkandexpendsthesamepowerforeachliS.

C.  ThecranedoesthesameworkforeachliSbutexpendsmorepowerforthefirstliS.

D.  ThecranedoesmoreworkinthesecondliSbutexpendsthesamepowerforeachliS.