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CU-Boulder PHYS 1240 - Lecture Notes

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1Would the amplitude on this graph mostlikely be…a) Velocity amplitudeb) Displacement amplitudec) Pressure amplituded) Wavelength amplitudee) None of the above5.1.1Position (m)Time (s)Which amplitude have we been mostconcerned with when studying music so far?(why?)Answer: The y-axis of the graph is labeled Position,so the amplitude must be measuring thedisplacement. (So far we have been moreconcerned with pressure amplitudes since soundis a pressure wave)5.1.2Which sound is stronger?a) The one on the leftb) The one on the rightc) They are the same strengthd) You can’t tellOver Pressure (N/m2)Time(s)Time(s)Over Pressure (N/m2)Answer: The term “Stronger” is a bit ambiguous, butif we associate it with the energy of the wave,then since energy is proportional to the square ofthe amplitude, then we would say that the waveon the left is stronger.5.1.3Looking at the following two graphswhat can you say about the energycarried by the wave?a)The energy is larger in I.b)The energy is larger in II.c)The energy is the same in bothd)You can’t tell anything about theenergy from these graphse)What does energy have to dowith music?Over Pressure (N/m2)Time (s)Over Pressure (N/m2)Time (s)I.II.Answer: The first graph must have higher energybecause the over pressure is higher, so thewave can do more work on something.5.1.4If you do overall positive work on asystem, what happens to the totalenergy of that system?a) Energy increasesb) Energy decreasesc) Energy is conserved, it does notchange.d) ??Answer: The system has positive work done on it, sothe energy of the system must have increased.Energy IS conserved, in this case that means that thething DOING the work is losing energy, and thesystem with work being done ON it is gaining. Theuniverse is neither losing nor gaining.(I am assuming that YOU are not part of the systemyou are doing work on!?)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 JAnswer: Energy is proportional to amplitude squared,so if you pull it 10 cm then you do (10)2 = 100times as much work as when you pull it 1 cm.Since it takes 2 J of energy to pull 1 cm, andsince pulling it 10 cm takes 100 times as muchenergy, the energy that it took is (2 J).100 = 200 J A B C D The above pictures represent wave frontsof a sound wave emitted from some source.Which source is moving at the speed ofsound?Answer: Picture A shows a stationary sound sourcewhile the others show a source moving to theright. B shows the source moving at the speedof sound since the sound waves are getting builtup on the far right. C shows a source movingslower than the sound speed and D is asupersonic source, going faster then the speedof sound.5.1.6A 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 aboveAnswer: Remember that: Power = Energy/time,which means Energy = Power.Time = (100 W).(2 s) = (100 J/s).(2 s) = 200 J(seconds cancel)5.1.6bIf 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 fineAnswer: The “Power Bar” is actually an “Energy Bar”.Your body runs off of the calories that you getfrom the food you eat and a calorie is a measureof energy (1 food Calorie = 4184 Joules).3BANG! 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) ??Answer: Power = Energy / Time = (1 J)/(0.2s) = 5 WAnswer: This is the same energy as the firecracker,but over a much longer time, so the power willbe less.5.1.6b2On Monday I walk slowly up the Gamowtower. This takes a certain amount ofenergy.On Tuesday, I run up as fast as I can.Compare the ENERGY and POWER on thetwo days:A) Energy expended is same, power ismore on TuesdayB) Power is same, more energy expendedon TuesdayC) Energy and power both the same onboth daysD) More energy and more power expendedon TuesdayE) ??Answer: On either day I do the same amount of workbecause I weight the same and I am lifting myweight over the same distance. (Energy is workdone = force*distance) On Tuesday I go faster, so since Power =Energy/Time, it takes more power, becauseTime is smaller5.1.9You 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 sameAnswer: Because we are at the same place (roughly) andwe are both listening to the exact same sound, the intensityof the sound that we each hear is exactly the same.Remember that intensity is a property of the sound.The intensities are the same so since Intensity =Power/Area, if our ears are the same size then the powerwe get must be the same. Since I listen for twice as longand since Power = Energy/Time, the energy that I get mustbe twice as big.(Makes sense! You keep getting more and more energyeach second, so if you listen longer, you get more energy)5.1.10You and I are standing next to each other, listening to theexact 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 sameAnswer: Because we are at the same place (roughly) andwe are both listening to the exact same sound, the intensityof the sound that we each hear is exactly the same.Remember that intensity is a proprerty of the sound.The intensities are the same so since Intensity =Power/Area, if my ears are twice as big, then the power Iget must be twice as big. (Makes sense. Think of two solarpanels next to each other on a roof. The BIGGER panelwill be in the exact same


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CU-Boulder PHYS 1240 - Lecture Notes

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