11A Analysingthe forces on a car
This carisstanding stillonafirm level surface. Therearetwoforcesacting upon it,weightand a reaction force from the ground (thegroundbeing capableofsupporting the weight of thecar).
1) Drawinthosetwoforces,showingthedirectionstheyareacting in.
2) The car isn’t moving:comparetheforcesintermsofsizeand direction.
11A Analysingthe forces on a car
This caristravelling forwardsalongalevelroad at a steady speedandina straightline.Various forcesareactinguponit.
1) Drawandlabelarrowsonthepictureto showthedirectionof:
a.The forceduetothemotor b.Weight
c.Airresistance
d.Friction in the movingparts
e.Reaction force fromtheground(theground beingstrongenoughtosupportit).
2) Thinkabout the sizeanddirectionoftheforces.
a.Compare the weightandthereactionforce. b.Compare theforceduetothemotor,
drivingitforwards,withthe forcesdue to air resistanceandfriction(rememberthe caristravellingat a steady speed).
c.Thinkabout the forces in Part B.Ifyou wantedthecartotravelfasterwhatcould youdotothe sizeofthoseforces?
11A Analysingthe forces on a car
This caris accelerating–itisspeedingup.
1) Whatcanyousayabout thesizeoftheforcefromtheengine compared with the size of the forcesfromfrictionand air resistance if its speed isincreasing?
2) Whatmight the designers have donetoenableittotravelatahigherspeed(rememberthat all Greenpower Challenge cars have the same sizemotors)?
Energy canbetransferred fromone place to another.
Engineers and scientists havetounderstandhowto managethosetransfersin order tomakebest use of it.
Greenpower Challengecars haveenergytransferredto thembythebatteriesbeing chargedup. Asthecarmoves, sothebatteriesrundown. Some of theenergy usedas
Designingfor energy efficiency
thecar movesisputtogood use; someofitiswasted.
Look at this list of waysinwhichenergyis transferredfromthebattery:
•Soundfrommovingparts
•Heatinthemotorandwiring
•Movingthecaralong theracetrack
•Pushingairinfrontofthecaroutoftheway
•Heatinmovingparts, such as axles
1) Dividetheseintotwolists,usefuland wasteful.
2) For each ofthewastefulones, suggestways inwhichtheycouldbereduced.
Sankeydiagram
Ausefulwayofshowingenergytransfersin a system (suchasacar)istouseadiagramcalled ‘Sankey’diagram.
Theinputontheleftisthetotalenergysupplied (inthis case theenergyinthefullycharged batteries).Theusefuloutputisshown horizontally,topright.Thewastedoutputis shownvertically, lowerright.Eitherofthe outputscanbesplitintoseparatearrowsifthere ismorethanone.(Ifweknewtheamountof energywecouldshowthis bythewidthofthe arrowsbutwedon’tinthis case).
3) Drawa‘Sankey’diagramtoshowtheenergy beingsuppliedbythebatteries,theuseful output(s)andthewastedoutput(s).Use your answersfromQ2toguideyou.
Totalenergyin
Useful energy out
4) Rememberthatthewidthofthearrows indicatestheamountofenergy.Whatdoes the‘Sankey’diagramlooklike foraless wastefulsystem?
5) Look at the Greenpowercarinthe illustration.What stepshavethedesigners takentomaketheircarmoreefficient?
Wasted energy out