Phys 1240: Sound and MusicLAST: outdoor sound, energyTODAY: intensity and loudnessNEXT: more on decibelsREAD:Finish Ch. 5 (can skip 1st partof 5.5, (p. 83) about amplitudes.)6.1 Don’t try to memorize - just thinkabout the physics of the importantpieces of your ear.6.2CAPA 5 due Thurs AFTER break.So… no help sessions this week.If you want to meet, email us!Reading q. eve r y Tues, as usual!A B C D The above pictures represent wavefronts of a sound wave emitted fromsome source. Which source is movingat the speed of sound?Measuring…FORCE => NewtonsWORK or ENERGY => JoulesRecall:Work = force * distance(So, 1 Joule = 1 Newton* 1 meter)Mass on a spring• If pull it twice as far => need twice the forceWork = force*distancePull it twice as far => “twice the force” * “twice the dist.” => FOUR times the work!Energy of oscillating objectsgrows like (amplitude)2CT 5.1x.1You pull a mass on a spring 1 cm andlet it go. Suppose it has 2 Joules (2 J)of energy.If you now pull it 10 cm and let it go,how much energy does it have?A) 2 JB) 10 JC) 20 JD) 100 JE) 200 JpowerIf energy CHANGES, we can ask“how rapidly”?POWER tells you this:(1 Joule of energy)/ (1 second)is called 1 Watt of power1 W = 1 J/secPower=Energy transfer / time taken5.1.6A light bulb has a power of 100 J/s (alsocalled 100 Watts). After being on for 2seconds how much energy has the bulbreleased?a) 100 Jb) 50 Jc) 200 Jd) 102 Je) None of the above5.1.6bIf the company had been run byphysics knurds, what should theyhave called the Power Bar?A) The Force BarB) The Pressure BarC) The Energy BarD) The Chemical BarE) The Power Bar is fineBANG! The sound wave from a fireworkcarries 1 J of energy (total!). Ifthe sound burst lasts .2 seconds, what isthe power of this sound wave?a) 1 Wb) .2 Wc) 5 Wd) 2 We) ??5.2.2A lawnmower runs for 10 minutes.Over that time, a total of 1 J of sound energy isreleased. How does the power of thelawnmower sound compare to that of thefirework sound?A) Greater than B) Less thanC) Equal to D) ??5.1.6b2On Monday I walk slowly up theGamow tower. This takes a certainamount of energy.On Tuesday, I run up as fast as I can.Compare the ENERGY and POWERon the two days:A) Energy expended is same, poweris more on TuesdayB) Power is same, more energyexpended on TuesdayC) Energy and power both the sameon both daysD) More energy and more powerexpended on TuesdayE) ??5.1.8The following is a graph of the energy ina system vs. time.As time goes on, what can you say aboutthe power going into the system?a) Power is increasing with timeb) Power is decreasing with timec) Power is steadyd) ???time (s)Energy (J)intensityEnergy often more interesting orimportant than pressure.Sound waves carry energy:energy flows through an AREA(and it flows over TIME.)intensityIntensity is Energy / (time * area) I = power / areaMeasured in Joules/(sec m2)(Which is also W/m2)It’s about the “flow of energy”, or “flux”If 1000 Joules of solar energy flowsthrough a 1 m2 window everysecond, we say“The intensity of light is 1000 W/m2Related to (not same as) “loudness”Depends on (but not linearly) “pressure amplitude”5.1.9You and I are standing next to each other,listening to the exact same sound.Our ears are identical.You listen for TWICE as long as I doHow does the power compare?How does the total energy received compare?How does the intensity received compare?A)You receive twice what I doB)You receive half what I doC)We receive the same5.1.10You and I are standing next to each other,listening to the exact same sound.Your ears are TWICE the area of mine!How does the power compare?How does the total energy received compare?How does the intensity received compare?A) You receive twice what I doB) You receive half what I doC) We receive the sameRemember, for any oscillation,energy grows like (amplitude)2.SoIntensity grows like
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