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Homework : Chapter 17

Homework : Chapter 17. 1,2,3,4,8, 11,13,19,21,27,28,31,34,35,37, 38, 39, 47,58,64.

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Homework : Chapter 17

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  1. Homework : Chapter 17 1,2,3,4,8, 11,13,19,21,27,28,31,34,35,37, 38, 39, 47,58,64

  2. 1. Suppose that you hear a clap of thunder 16.2 s after seeing the associated lightning stroke. The speed of sound waves in air is 343 m/s and the speed of light in air is 3.00  108 m/s. How far are you from the lightning stroke? Since Count the seconds after seeing the lightening and divide by 3 to get the approximate distance in km. It is about

  3. Find the speed of sound in mercury, which has a • bulk modulus of approximately 2.80x1010 N/m2 and a density of 13 600 kg/m3.

  4. 3. A flowerpot is knocked off a balcony 20.0 m above the sidewalk and falls toward an unsuspecting 1.75-m-tall man who is standing below. How close to the sidewalk can the flower pot fall before it is too late for a warning shouted from the balcony to reach the man in time? Assume that the man below requires 0.300 s to respond to the warning. Sound takes this time to reach the man: so the warning should be shouted no later than before the pot strikes. Since the whole time of fall is given by the warning needs to come into the fall, when the pot has fallen to be above the ground by

  5. 4.The speed of sound in air (in m/s) depends on temperature according to the approximate expression v= 331.5 + 0.607TC where TCis the Celsius temperature. In dry air the temperature decreases about 1°C for every 150 m rise in altitude. (a) Assuming this change is constant up to an altitude of 9 000 m, how long will it take the sound from an airplane flying at 9 000 m to reach the ground on a day when the ground temperature is 30°C? (b) What If? Compare this to the time interval required if the air were a constant 30°C. Which time interval (9000/150) = -60 C so T = -30 C a) at 9000 m, ∆T = -1C Since v depends on T and T depends on the height x and since we need to Know v=v(t) where t is the time, we apply the chain rule twice: b) t = h / v = 9000/ [331.5+0.607(30) ] = 25.7 s

  6. 8. An ultrasonic tape measure uses frequencies above 20 MHz to determine dimensions of structures such as buildings. It does this by emitting a pulse of ultrasound into air and then measuring the time for an echo to return from a reflecting surface whose distance away is to be measured. The distance is displayed as a digital read-out. For a tape measure that emits a pulse of ultrasound with a frequency of 22.0 MHz, • what is the distance to an object from which the echo pulse returns after 24.0 ms when the air temperature is 26oC? • (b) What should be the duration of the emitted pulse if it is to include 10 cycles of the ultrasonic wave? • What is the spatial length of such a pulse? The sound speed is (a) Let t represent the time for the echo to return. Then (b) Let represent the duration of the pulse: c)

  7. 11.A sinusoidal sound wave is described by the displacement wave function s(x, t) = (2.00 µm)cos[(15.7 m–1)x–(858 s–1)t] (a)Find the amplitude, wavelength, and speed of this wave. (b) Determine the instantaneous displacement from equilibrium ofthe elements of air at the position x= 0.050 0 m at t= 3.00 ms. a) b)

  8. 13. Write an expression that describes the pressure variation as a function of position and time for a sinusoidal sound wave in air, if = 0.100 m and Pmax = 0.200 N/m2.

  9. 19. Calculate the sound level in decibels of a sound wave that has an intensity of 4.00 W/m2.

  10. 21. The intensity of a sound wave at a fixed distance from a speaker vibrating at 1.00 kHz is 0.600 W/m2. (a) Determine the intensity if the frequency is increased to 2.50 kHz while a constant displacement amplitude is maintained. (b) Calculate the intensity if the frequency is reduced to 0.500 kHz and the displacement amplitude is doubled. At the frequency is increased by a factor of 2.50, so the intensity (at constant ) increases by Therefore, b)

  11. 27. The power output of a certain public address speaker is 6.00 W. Suppose it broadcasts equally in all directions. (a) Within what distance from the speaker would the sound be painful to the ear? (b) At what distance from the speaker would the sound be barely audible? We have assumed the speaker is an isotropic point source. b) We have assumed a uniform medium that absorbs no energy.

  12. 28.Show that the difference between decibel levels 1 and 2 of a sound is related to the ratio of the distances r1and r2 from the sound source by 30. A loudspeaker is placed between two observers who are 110 m apart, along the line connecting them. If one observer records a sound level of 60.0 dB, and the other records a sound level of 80.0 dB, how far is the speaker from each observer? Let and be the distance from the speaker to the observer that hears 60.0 dB and 80.0 dB, respectively. Use the result of problem 28, So, and

  13. 31. Two small speakers emit sound waves of different frequencies. Speaker A has an output of 1.00 mW and speaker B has an output of 1.50 mW. Determine the sound level (in dB) at point C (Fig. P17.31) if (a) only speaker A emits sound, (b) only speaker B emits sound, (c) both speakers emit sound. We presume the speakers broadcast equally in all directions. a)

  14. b) (c) c)

  15. 34. A fireworks rocket explodes at a height of 100 m above the ground. An observer on the ground directly under the explosion experiences an average sound intensity of 7.00  10-2 W/m2 for 0.200 s. • What is the total sound energy of the explosion? • What is the sound level in decibels heard by the observer? a) b)

  16. 35.As the people sing in church, the sound level everywhere inside is 101 dB. No sound is transmitted through the massive walls, but all the windows and doors are open on a summer morning. Their total area is 22.0 m2. (a) How much sound energy is radiated in 20.0 min? (b) Suppose the ground is a good reflector and sound radiates uniformly in all horizontal and upward directions. Find the sound level 1 km away. The sound intensity inside the church is given by We suppose that sound comes perpendicularly out through the windows and doors. Thus the radiated power is The energy radiated in 20 min is E=Pt=332J b) If the ground reflects all sound energy headed downward, the sound power, P = 0.277W covers the area of a hemisphere. One kilometer away, this area is A = 2πr2= 2π106m2. The intensity level at this distance is

  17. 37.A train is moving parallel to a highway with a constant speed of20.0 m/s. A car is traveling in the same direction as the train with a speed of 40.0 m/s. The car horn sounds at a frequency of 510 Hz and the train whistle sounds at a frequency of 320 Hz. • When the car is behind the train what frequency does an occupant of the car observe for the train whistle? • (b) After the car passes and is in front of the train, what frequency does a train passenger observe for the car horn?

  18. Expectant parents are thrilled to hear their unborn baby’s heartbeat, revealed by an ultrasonic motion detector. Suppose the fetus’s ventricular wall moves in simple harmonic motion with an amplitude of 1.80 mm anda frequency of 115 per minute. • Find the maximum linear speed of the heart wall. Suppose the motion detector in contact with the mother’s abdomen produces sound at 2000 000.0 Hz, which travels through tissue at 1.50 km/s. • (b) Find the maximum frequency at which sound arrives at the wall of the baby’s heart (c) Find the maximum frequency at which reflected sound is received by the motion detector. By electronically “listening ”for echoes at a frequency different from the broadcast frequency, the motion detector can produce beeps of audible sound in synchronization with the fetal heartbeat

  19. 39.Standing at a crosswalk, you hear a frequency of 560 Hz from the siren of an approaching ambulance. After the ambulance passes, the observed frequency of the siren is 480 Hz. Determine the ambulance’s speed from these observations. Approaching car Receding car

  20. 47.A supersonic jet travelling at Mach 3.00 at an altitude of 20000 m is directly over a person at time t= 0 as in (Fig). a) How long will it be before the person encounters the shock wave? (b) Where will the plane be when it is finally heard? (Assume the speed of sound in air is 335 m/s.)

  21. 64. A jet flies toward higher altitude at a constant speed of 1 963 m/s in a direction making an angle with the horizontal (Fig. P17.64). An observer on the ground hears the jet for the first time when it is directly overhead. Determine the value of if the speed of sound in air is 340 m/s. The shock wavefront connects all observers first hearing the plane, including our observer O and the plane P, so here it is vertical. The angle that the shock wavefront makes with the direction of the plane’s line of travel is given by Using the right triangle CPO, the angle is seen to be

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