phys124s11-l07

phys124s11-l07 - 1 RG Physi cs& Ast r onomy Physics 124...

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Unformatted text preview: 1 RG, Physi cs & Ast r onomy Physics 124 Spring 2009 Physics 124 Spring 2011 L7 4 Mar 2011 Lecture 6 Summary Power: P = E total l v = 1 2 v 2 A 2 = 1 2 T 2 A 2 speed: v = T / wave equation: ∂ 2 y ∂ x 2 = k 2 2 ∂ 2 y ∂ t 2 = 1 v 2 ∂ 2 y ∂ t 2 y = Acos kx − t Generally f x,t f x − vt v = k = T = f k = 2 / v y = dy dt =− Asin kx − t vs. Intensity: I r = P 4 r 2 2 RG, Physi cs & Ast r onomy Physics 124 Spring 2009 Physics 124 Spring 2011 L7 4 Mar 2011 Today: Sound Waves Ch 16.1-3, 16.8-9 bouncing biscuit demo 3 RG, Physi cs & Ast r onomy Physics 124 Spring 2009 Physics 124 Spring 2011 L7 4 Mar 2011 Sound Wave Introduction traveling wave with longitudinal displacement sound is not a wind or breeze − gas atoms oscillate back and forth audible sound ranges from ≈ 20 Hz to 20 kHz Recall for string v 2 = T = tension mass density = restoring force inertia For gas: v 2 = B Recall bulk modulus B = P V / V and = density so we have pressure mass = restoring force inertia again simulation: Colorado JAVA sound.jar v air ≈ 340 m / s λ = v f = 340 20 = 17 m as before, e.g. 4 RG, Physi cs & Ast r onomy Physics 124 Spring 2009 Physics 124 Spring 2011 L7 4 Mar 2011 Points for Demo No air, no sound − need a medium Speed v is a property of the medium speed = wavelength x frequency: v = f Particles displaced along direction of wave motion leading to alternating regions of high and low pressure 5 RG, Physi cs & Ast r onomy Physics 124 Spring 2009 Physics 124 Spring 2011 L7 4 Mar 2011 Today: Sound Waves Ch 16.1-3, 16.8 For gas: v 2 = B Recall bulk modulus B = P V / V and = density so we have pressure mass = restoring force inertia again simulation: Colorado JAVA sound.jar What is the wavelength of 500 Hz sound? a: 0.1 m b: 0.1 − 0.5 m c: 0.5 − 1 m d: 1 − 2 m e: 2 m 6 RG, Physi cs & Ast r onomy Physics 124 Spring 2009 Physics 124 Spring 2011 L7 4 Mar 2011 Today: Sound Waves Ch 16.1-3, 16.8 For gas: v 2 = B Recall bulk modulus B = P V / V and = density so we have pressure mass = restoring force inertia again simulation: Colorado JAVA sound.jar How long will it take the 500 Hz sound to go 5 m? a: 1 ms b: 1 − 10 ms c: 10 − 100 ms d: 100 − 1000 ms e: 1 s 7 RG, Physi cs & Ast r onomy Physics 124 Spring 2009 Physics 124 Spring 2011 L7 4 Mar 2011 Speed of sound waves v air ≈ 343m / s common more precise approximation: v air = 331 m / s 1 T c in o C 273 o C He less dense than air, lighter atoms move faster, v He v air higher frequency sounds Liquids have higher v than gases solids have speed v = Y = Young's modulus density v Al = 6.4 km / s v water ≈ 1500m / s f = 1 kHz = 0.33 m text page 534: v air ≈ 344m / s Not only this: bulk modulus varies 8 RG, Physi cs & Ast r onomy Physics 124 Spring 2009 Physics 124...
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phys124s11-l07 - 1 RG Physi cs& Ast r onomy Physics 124...

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