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Home , F, Piano wire, Transverse wave, Ultrasound

Phys 197 Homework Solution 15A

Q12. The four strings on a violin have different thicknesses, but are all under approximately the same tension. Do travel faster on the thick strings or the thin strings? Why? How does the fundamental vibration compare for the thick versus the thin strings? ————– Since = /µ, where F is the tension and µ is the linear mass density (in kg/), waves p travel faster on the thin strings. The period is proportional to the time for a to travel the length of the , so the thin strings have a shorter period and hence a higher frequency.

Q21. A musical interval of an octave corresponds to a factor of 2 in frequency. By what factor must the tension in a guitar or violin string be increased to raise its pitch one octave? To raise it two octaves? Explain your reasoning. Is there any danger in attempting these changes in pitch? ————– Doubling the frequency calls for doubling the wave speed. Since v = F /µ, v = 2v = 0 p 0 1 0 2 F /µ = 4F /µ. The tension must be increased to 4F , a factor of 4. Going up two octaves p 0 p 0 0 would be a factor of 16 in tension. like a good chance of broken strings or other parts.

P1. The speed of in air at 20◦ is 344 m/ (a) What is the of a sound wave

with a frequency of 784 Hz, corresponding to the note G5 (the just above the treble staff) on a , and how many milliseconds does each vibration take? () What is the wavelength of a sound wave one octave higher than the note in part (a)? ————– (a) λ = v/f = (344 m/s)/784 Hz) = 0.439 m. = 1/f = 1/(784 Hz) = 0.001276 s = 1.276 ms. (b) The wavelength is halved: λ = 0.219 m.

P4. Ultrasound Imaging. Sound having above the range of human (about 20,000 Hz) is called ultrasound. Waves above this frequency can be used to penetrate the body and to produce images by reflecting from surfaces. In a typical ultrasound scan, the waves travel through body tissue with a speed of 1500 m/s. For a good, detailed image, the wavelength should be no more than 1.0 mm. What frequency sound is required for a good scan? ————– Since v = λf, f = v/λ = (1500 m/s)/(10−3 m) = 1.5 MHz. That’s the same frequency as “1500” on your AM radio dial (in sound waves, versus electromagnetic waves).

P7. Transverse waves on a string have wave speed 8.00 m/s, amplitude 0.0700 m, and wavelength 0.320 m. The waves travel in the -direction and at t = 0 the end of the string has its maximum − upward displacement. (a) Find the frequency, period, and wave number of these waves. (b) Write a wave function describing the wave. (c) Find the transverse displacement of a particle at x =0.360 m at time t =0.150 s. () How much time must elapse from the instant in part (c) until the particle at x =0.360 m next has maximum upward displacement? ————– (a) f = v/λ =(8m/s)/(0.32 m) = 25.0 Hz. T = 1/f = 0.0400 s. = 2π/λ = 19.64 m−1. (b) The standard formula for a wave is (x, t) = A cos(kx ωt). We have A and k, but still need ω. ± ω = 2πf = (6.28 (rad/s)/Hz)(25 Hz) = 157.1 rad/s. Note the units I ascribe to the 2π factor. y(x, t) = (0.0700 m) cos(19.64 x + 157.1 t). (Suppressing the units). The plus sign is chosen to correspond to the wave traveling in the negative x direction. (c) Plug: y(0.36 m, 0.15 s) = (0.7 m) cos(7.0704 + 23.565) = 0.0497 m. Keep extra sigdigs. (d) Calculate the total phase (argument of the cosine), which is of 7.0704 rad + 23.565 rad = 30.635 rad. Divide this by 2π to see how many cycles have elapsed: Number of cycles = (30.635 rad)/(2π) = 4.8757. So it needs just 0.1243 of a period to re-attain max: 0.1243 T = 0.1243 (0.0400 s) = 0.0497 s. This differs slightly from· the answer in the back of the book due to the extra sigdig. It shows that a very small error (or change) in k or ω accumulates over a number of cycles.

P8. A certain is described by x t y(x,t)=(6.50 mm) cos 2π ³28.0cm − 0.0360 s´ Determine the wave’s (a) amplitude; (b) wavelength; (c) frequency; (d) speed of propagation; () direction of propagation. ————– (a) Amplitude is absolute value of coefficient: A = 6.50 mm. (b) Wavelength: We know that k = 2π/λ in general, and that k = 2π/(28.0 cm) in this instance. So, λ = 28.0 cm. (c) Frequency: Similarly, ω = 2π/T in general, and ω = 2π/(0.0360 s) in this instance, so T = 0.360 s. Frequency is the reciprocal: f = 1/T = 1/(0.360 s) = 27.8 Hz. (d) Speed of propagation: v = λ/f = (0.28 m)/(0.0360 s) = 7.78 m/s. Converted to m/s as a matter of habit, in case it gets plugged into something else. (e)Direction of propagation: +x, since there is a negative sign between kx and ωt.

P15. One end of a horizontal rope is attached to a prong of an electrically driven that vibrates the rope transversely at 120 Hz. The other end passes over a pulley and supports a 1.50-kg mass. The linear mass density of the rope is 0.0550 kg/m. (a) What is the speed of a transverse wave on the rope? (b) What is the wavelength? (c) How would your answers to parts (a) and (b) change if the mass were increased to 3.00 kg? ————– (a) v = F/µ = (1.5 kg)(9.8m/s2)/(0.0550 kg/m) = 267.3m2/s2 = 16.3 m/s. If you p p p forgot to multiply by g, the units on the RHS would be √m; since this is not the unit for velocity, you would know to double-check. (b) v = λf λ = v/f = (16.3m/s)/(120 Hz) = 0.136 m. (c) v would increase⇒ by a factor of √2 = 1.414 to 23.1m/s and wavelength would increase by the same factor to 0.192 m.

P42. A piano tuner stretches a steel with a tension of 800 . The steel wire is 0.400 m long and has a mass of 3.00 g. (a) What is the frequency of its fundamental mode of vibration? (b) What is the number of the highest that could be heard by a person who is capable of hearing frequencies up to 10,000 Hz? ————–

(a) The fundamental is f1 of Eq 15.32: f1 = v/2L. The v in the equation is the speed of the wave on the string, given by v = F/µ = (800 N)/[(0.003 kg)/(0.4 m)] = 326.6m/s. p p Note that converting grams to kg is essential.

f1 = v/2L = (326.6m/s)/(2 0.4m)= 408.3 Hz. (b) 10 000 Hz/408.3 Hz = 24.5,· so up to the 24th harmonic at 9800 Hz.

P49. Guitar String. One of the 63.5-cm-long strings of an ordinary guitar is tuned to produce the note B3 (frequency 245 Hz) when vibrating in its fundamental mode. (a) Find the speed of transverse waves on this string. (b) If the tension in this string is increased by 1.0%, what will be the new of the string? (c) If the in the surrounding air is 344 m/s find the frequency and wavelength of the sound wave produced in the air by the vibration of the string. How do these compare to the frequency and wavelength of the on the string? ————–

(a) f1 = v/2L, so v = 2Lf1 = 2 (0.635 m)(245 Hz) = 311 m/s. Note the necessity to convert centimeters to meters. · (b) If tension is multiplied by 1.01, the speed v is to be multiplied by √1.01 = 1.005. The fundamental is directly proportional to the velocity, so ′ f1 = 1.005 f1 = (1.005)(245 Hz) = 246.2 Hz. (c) point is that the frequency is the physical shaking, which has to be the same. So:

fin air = 245 Hz. The fundamental on the string is such that the length of the string is one-half a wavelength:

λon string = 2L = 1.270 m. In air, use the formula v = λf: λ = v/f = (344 m/s)/(245 Hz) = 1.404 m.