PHYSICS NOTES. SUBJECT: Physics LEVEL: Higher TEACHER: Pat Doyle. The Institute of Education Topics Covered: Sound & Waves

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1 PHYSICS NOTES The Institute Eduatin 208 SUBJECT: Physis LEVEL: Higher TEACHER: Pat Dyle Tpis Cvered: Sund & Waves Abut Pat: Pat graduated with a Masters Degree in Physis in 986 and has taught Physis in the Institute r ver 30 years. He has been a regular ntributr t Siene Plus, a Siene and Mathematis publiatin r the Leaving Cert. He reently -wrte Explring Siene, Explring Siene Wrkbk and Revise Wise Siene r the Junir Certiiate.

2 Sund and Waves The Dppler Eet Yu are standing n the side a rad listening t the sund rm a siren n the r a statinary ar that is 500 m dwn the rad. The ar starts t mve twards yu and yu ntie that the requeny the sund is dierent (higher). The ar passes by yu and mves away rm yu. The requeny the sund is dierent again (lwer). This is due t the Dppler eet. Deinitin Dppler eet: The Dppler eet is the apparent hange in the requeny a wave due t the relative mtin between sure and bserver. Explanatin Dppler eet: Sure mving twards Statinary bserver Sure mving away bserver rm bserver (i) wavernts lser tgether (ii) bserved wavelength smaller (iii) bserved requeny higher (i) wavernts urther apart (ii) bserver wavelength lnger (iii) bserved requeny lwer The bserved wavelength and requeny hange as the sure mves past bserver!!!!!!!!!!

3 Demnstratin (labratry) experiment r the Dppler Eet: A battery pwered eletrni buzzer is attahed t a string 50 m lng. The emitted requeny is set at 300 Hz. The buzzer is spun in irular mtin n the end the string quite ast. An bserver standing 2 m away will ntie the requeny rm the buzzer hange as it rtates. The bserved requeny hanges as the buzzer mves twards and then mves away rm the persn. Maths Dppler eet: (! u P. 59! = bserved requeny; = requeny sure; = speed wave; u = speed sure) Use the when sure mves twards the bserver Use the + when the sure mves away rm the bserver Nte: It is very easy t nuse! and. I prpse we use the llwing s u ( = bserved requeny; s = requeny sure ) Sample questin : A plie ar is travelling at 25ms and has a siren emitting a sund at a requeny 480 Hz. What is the requeny deteted by a statinary bserver (i) as the ar apprahes (ii) as the ar mves rm the persn. ( speed sund in air = 340ms ) Slutin: (i) s u (ii) s u Hz 447 Hz

4 Sample questin 2: A train s whistle emits a ntinuus nte requeny 640 Hz as it apprahes a persn standing near the trak. T the persn the requeny appears t be 720 Hz. Calulate the speed the train. ( speed sund in air = 340ms ) Slutin: the train apprahes the persn s use s u s u u = u = = u 37.8ms = u Sample questin 3: A whistle emitting a nte khz is whirled in a hrizntal irle n the end a string.2 m lng at a nstant angular speed 50 rad s. What are the highest and lwest requenies heard by a persn standing sme distane. away. ( speed sund in air = 340ms ). Slutin: step : hange angular speed t linear speed v r v.2 50 v 60ms Step 2: whistle mving twards the bserver s u Step 3: whistle mving away rm bserver s u Hz 850 Hz

5 Sample questin 4: The yellw line emitted by a Helium disharge tube in the labratry has a wavelength 587 nm. The same yellw line in the helium spetrum a star has a measured wavelength 590 nm. (i) what an yu dedue abut the mtin the star (ii) alulate the speed the star. 8 ( speed light = 3 0 ms ) Slutin: (i) the bserved wavelength is lnger therere the star mves away rm the earth. (ii) hange the wavelengths t requenies True requeny ( sure ) = s = Observed requeny = = = = Hz Hz Nw rearrange s u t get u u = u = s ms u Cheking the speed a ar: Appliatins the Dppler eet: An eletrmagneti wave knwn requeny s is direted at a mving ar. The speed this wave is als knwn as all eletrmagneti waves the same speed. The value 8 the speed the wave is 3 0 ms The wave releted the ar will have a dierent requeny t the inident wave. This releted requeny is measured eletrnially. The values s, s u and are nw all knwn. We alulate the speed the ar u using

6 Studying the stars: I a star is mving twards the earth the requeny the light rm the star appears higher. The wavelength appears smaller. There is a shit twards the vilet end the spetrum I a star is mving away rm the earth the requeny the light rm the star appears lwer. The wavelength appears lnger. There is a shit twards the red end the spetrum. red shit. The atual speed a star an be alulated using ur Dppler rmula. Nte: Other uses the Dppler eet wuld inlude Bld lw measurement ( ehardigram ) Cheking the heart beat a etus.

7 Fundamental mde vibratin: Resnane and Strethed Strings I a string is ixed at bth ends and pluked in the entre the mst simple r undamental type vibratin is shwn in the abve diagram. Fr the undamental mde the string vibrates at its lwest requeny Harmnis: 2 nd harmni 3 rd harmni. Harmnis are multiples the undamental mde vibratin The diagram abve shws the string vibrating in the send and the third harmni The vertnes are all the harmnis exept the irst harmni. Pith: Pith is related t the requeny a sund. The higher the requeny the higher the pith pith is t sund as lur is t light Quality: vilin pian. The same nte played n tw dierent instruments des nt sund the same e.g. the pian and the vilin. The quality a sund depends n the number and intensity the harmnis (vertnes) present Ludness: The ludness a sund depends n the amplitude the vibratin the sund wave. Ludness is subjetive. Charateristis a sund: The harateristis a sund are pith, quality and ludness. These are the three ways in whih sunds dier.

8 Natural requeny: When a system that is apable vibrating is made t vibrate it will d s at its natural requeny. When a strethed elasti band is pluked in the middle it will vibrate at its natural requeny Fred requeny: When an external vibratin re ats n a system that is apable vibrating the external re prvides the red requeny. Tuh a vibrating tuning rk t a strethed string. The string vibrates at the same requeny as the tuning rk. The requeny the tuning rk is the red requeny. Resnane: Resnane happens when a vibrating system respnds with maximum amplitude t a red requeny. Resnane happens when the red requeny is equal t the natural requeny the vibrating system. An example wuld be a singer wh shatters a wine glass. The wine glass is the system apable vibrating. It has a natural requeny. The sund rm the singer prvides the red requeny. When red requeny equals the natural requeny resnane happens. The glass shatters. Demnstratin (labratry) resnane: (Bartn s pendulums) A number pendulums are arranged as shwn abve. Pendulum is made swing in and ut the plane the page. All the pendulums start t swing a little but pendulum 5 swings mst. Pendulums and 5 have the same length and therere the same natural requeny. Energy is transerred bak and rth between the pendulums the same natural requeny.