Describing motion. Unit 6.1. context. Fact File. Distance and displacement. Science

Transcription

1 Uni 6. conex Eeryhing in he uniere i in moion all he ime. We are moing a 00 km/h a he Earh pin on i axi and orbi he Sun. The Sun orbi he cenre of our galaxy, he Milky Way. Eer ince he Big Decribing moion Bang, our galaxy ha been moing away from all he oher galaxie. On a much maller cale, molecule are whizzing around in he air, and elecron are orbiing inide hem. Underanding moion i one of he key o underanding he world around u, and cieni hae deeloped clear way of decribing moion. Fig 6.. Thi kier i moing o fa ha he camera canno keep up. Thi make he kier appear a a blur. Diance and diplacemen To decribe a journey, mo people would menion he diance raelled and he ime i ook. When phyici decribe a journey, hey ue ime much a oher people would. Howeer, hey ue wo differen erm o decribe how far you hae raelled: diance hi meaure he acual diance raelled. Diance doe no inole he direcion in which you moe a any ime in your journey diplacemen hi meaure how far you end up from where you ared, and in which direcion (up, lef, norh, oward he window ec.). Diplacemen i diance bu wih direcion. Diance and diplacemen are meaured in he ame uni. Any lengh uni can be ued, bu diance and diplacemen are uually conered ino mere (uni ymbol m) for calculaion. Mo uden moe coniderable diance eery day: hey rael o and from chool, moe from cla o cla, around heir uburb or home own and wihin heir home. 06 Fig 6.. Diance i how far you rael. Diplacemen i how far, and in which direcion, you end up from where you ared. When hey reurn home, hey are back a he poin hey ared ou from ha morning. Their diplacemen i zero. Fac File Diance and diplacemen Symbol in formulae: diance ha no direcion diplacemen ha direcion Uni: mere Uni abbreiaion: m Time Symbol in formulae: Uni: econd Uni abbreiaion:

2 Speed and elociy Speed Car peed i meaured coninuouly by he peedomeer in kilomere per hour (km/h or kmh ). Thi i a meaure of he car inananeou peed or i peed a any momen in i rael. Speed i he rae a which diance i coered he amoun of diance coered for each uni of ime. Commonly ued uni of ime are hour (uni ymbol h) and econd (). If he peed of a car i meaured in kilomere per hour (km/h), hen he car would rael ha number of kilomere in one hour (auming ha he car kep moing a i curren peed). Thi mean ha a car raelling a counry freeway a 0 km/h will rael 0 km eery hour i doe o. Anoher common uni ued o meaure peed i mere per econd (m/ or m ). Scieni uually coner all peed and elociie ino mere per econd for calculaion. To coner kilomere per hour ino mere per econd, diide by.6. To coner mere per econd o kilomere per hour, muliply by.6: Ȗ.6 km/h m/.6 You do no alway hae a peedo or radar gun wih you o meaure inananeou peed. Some imple meauremen, howeer, allow you o calculae aerage peed: diance raeled aerage peed ime aken or Fac File Symbol and uni Each of he phyical quaniie ha are ued o decribe moion ha boh a ymbol and a uni. For example, he ymbol ued for ime i bu he uni in which ime i meaured i he econd (). Slighly confuingly, he ymbol ued for diplacemen i and he uni for diplacemen i he mere (m). Ialic are ued for ymbol bu no for uni, which make i eaier o diinguih beween hem. I imporan o ue he ymbol and uni correcly for clear cienific communicaion. We ue a mall arrow aboe he erm for quaniie where direcion i imporan, uch a diplacemen ( ) and elociy ( ), ince oherwie hey ue he ame ymbol a diance () and peed (). Fig 6.. Police radar gun meaure inananeou peed. Tha low! The peed limi for car in France wa km/h in 89. Originally all car in Grea Briain were required o hae a man walking in fron of hem wih a red flag o aler hore rider! In 896 he peed limi wa raied o 0 km/h and, in 90, o km/h. The fir Auralian peeding icke wa gien o Tamanian George Inne, who wa recklely driing a car hrough Sydney a km/h! Fac File Speed and elociy Symbol in formulae: peed ha no direcion elociy ha direcion Uni: mere per econd Uni abbreiaion: m/ or m If a chool bu ook half an hour o rael 0 kilomere o chool, hen i aerage peed would be: 0 0 km/h 0. Thi eem low, bu i an aerage of all he inananeou peed he bu did on i journey. The bu wen faer han 0 km/h, bu alo opped a raffic ligh and bu op. I alo had o reduce i peed hrough chool zone and hopping area. Uni 6. 07

3 Decribing moion Fig 6.. Since direcion i imporan a well a peed, weaher repor ell you he wind elociy, e.g. 0 kno ouh-ea. A kno i a nauical mile per hour, abou.8 km/h, o a 0-kno wind i blowing a abou 6 km/h. Fig 6.. Muli-flah and compoie high-peed phoograph pli a complex moion ino a erie of hor ime frame. The ime-ineral for each frame i o hor ha i aerage peed i cloe o he inananeou peed for ha frame. Velociy A weaher repor of 60 km/h wind gu i uele o pilo, ailor, urfer and people fihing unle hey alo know he wind direcion. Velociy i peed in a gien direcion. Velociy ha he ame relaionhip o peed ha diplacemen ha o diance hey are meaured in he ame uni, bu add informaion abou direcion. Wind moemen i uually aed a a elociy for example, a 60 km/h wind coming from he ouh. aerage elociy Meauring moion diplacemen ime Prac p. Aerage are ueful bu ell lile abou wha i acually happening a a paricular momen. If he diance or ime choen for he aerage i mall, howeer, aerage and inananeou peed become cloer o each oher. A runner migh be imed a compleing he 00 mere prin in econd, bu i would be beer o meaure he ime aken o run pa marker paced a, for example, 0 mere. The aerage peed of each ecion would how any change ha happened along he way. Spacing of one mere would be een beer. In he laboraory, he moion of an objec can be meaured in a number of way depending on he equipmen aailable. Daalogger here are a number of deice ha will meaure he diance, ime, peed and direcion of moemen of an objec, he daa being ranmied by infra-red beam or cable o a compuer for analyi by pecialied ofware. The informaion obained i accurae and can be eaily manipulaed ino oher form uch a graph and alue for acceleraion and force. Muli-flah phoography a robocope i flahed ono he moing objec and a camera or ideo on long expoure cache i image eery ime he robe ligh flahe. Diance can be meaured accuraely off he image and ime can be calculaed from he flah rae of he robe. The adanage of muli-flah phoography i ha i record differen moion wihin he objec, uch a he moemen of arm and leg. 08

4 Graphing moion Diance ime graph Graph are ery ueful in repreening he moion of an objec raelling in a raigh line. Diance ime graph how he oal diance raelled by an objec a ime progree. Time i alway placed on he horizonal axi and diance on he erical. Seep graph indicae ha he objec i coering more diance and raelling faer han flaer graph. A horizonal graph indicae no moemen a all: he objec i a re or aionary. The lope or gradien of a diance ime graph gie u he objec peed. hallow graph indicae ha he objec i raelling lowly eep diance ime graph indicae ha he objec i raelling fa horizonal graph indicae no moemen a all: he objec i a re Tha fa! carbon paper dik 0 do are produced eery econd. The pace beween do herefore repreen one fifieh of a econd (or 0.0 ) Conan peed: do are conanly paced Diance (m) o AC power pack To calculae gradien pick any poin 6 rie = 6 gradien = rie run = 6 = m/ Acceleraing: pacing beween do increae run = 0 0 Deceleraing: pacing beween do decreae 6. paper icker-ape Vibraing arm: hi arm ibrae a a rae of 0 Hz. I ac a a hammer, hiing he ape 0 ime eery econd Uni High-peed compoie phoography many camera will ake a erie of rapid image ha can hen be combined o form a ingle image. Thi mehod produce a imilar image o ha of muliflah phoography. If he rae i known, hen peed can eaily be calculaed from he image. Ticker-imer an older deice known a a ickerimer break moemen ino a erie of mall ineral. I proide a way of accuraely meauring diance raelled and ime aken and proide he daa from which aerage peed can be calculaed. A mall elecric hammer rike a piece of carbon paper a he ame frequency a i AC power upply, a a rae of 0 herz (0 Hz) or 0 ime a econd. Moion i recorded a do on a rip of paper ha pae under he hammer. Unlike mo daalogger, ickerprac imer can only meaure raigh-line p. moion in one direcion. Ruler and opwache ince aerage peed only need he diance raelled and he ime aken, ruler, ape meaure and opwache can proide daa ha will gie you a rough oeriew of an objec moion. Time () Meauremen are only a accurae a he deice ha meaure hem, and fauly equipmen will neer gie accurae meauremen. Thi wa paricularly rue when a drier in Belgium wa fined afer a radar gun meaured hi peed a 00 km/h! Fig 6..7 The lope or gradien of a diance ime ar graph gie he peed of an objec. The eeper a diance ime graph, he faer he objec i going. Fig 6..6 Alhough ueful, he icker-imer can only record moion in a raigh line in one direcion. 09

5 Decribing moion Speed ime graph A graph of peed again ime gie anoher picure of wha i happening in he moion of an objec. A before, ime i placed on he horizonal axi. If he objec i geing faer, he graph rie. If i i lowing, he graph fall. Conan peed gie a fla graph. The area under a peed ime graph gie he diance ha he objec ha raelled up o ha poin. You can coun he quare or ue area-formulae o find he diance raelled. Prac p. A riing peed ime graph indicae ha he peed of he objec i geing faer The area under a peed ime graph gie he diance ha he objec ha raelled up o ha poin A fla graph indicae ha he objec i raelling a a conan peed The falling graph indicae ha he objec i lowing Area can be calculaed by couning he quare or by uing area-formulae. The area under he graph here i 6 8. The objec ha moed m Calculaing diance The aerage peed formula can be rearranged o gie anoher ueful formula: diance peed ime or A car raelling a an aerage peed of 0 m/ for econd will hae raelled a diance of: 0 00 m Human do no repond immediaely o emergencie, bu ake up o. econd o reac. Thi i heir reacion ime. Thi mean ha when in a car, a drier will no begin braking unil well afer hey ee an emergency. Meanwhile he car i raelling fa oward i. To calculae he diance a car rael while he drier reac, he peed mu be conered ino m/ o mach he uni ued for ime. Aume a car i being drien a 60 km/h (6.7 m/) by a drier wih a reacion ime of. econd. The diance he car rael before he drier brake i hen: m Thi i equialen o fie o ix car lengh. A drier who i diraced (uing a mobile phone, changing a radio aion or who ha conumed alcohol) may ake a long a hree econd o reac. Workhee 6. Diance ime graph Speed(m/) area 6 area Time() Fig 6..8 The oal diance raelled i he area under a peed ime graph. The area here i 6 e 8 q. The objec ha moed mere. Drag-and-drop Don een hink abou opping! Prac p. In abou 700 BCE, King Sanherib of Ayria buil a road from hi capial, Nineeh, o nearby emple. I wa o wide ha i would hae been equialen o a modern freeway of 8 lane! The king wa juifiably proud of hi road and didn wan i poiled by chario parked along i. Deah wa he penaly for doing o, wih offender being impaled on pike! Prac p. 6 0