Post on 10-Mar-2018
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.