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Questions 569 The superposition principle specifies that when two or more waves move through a medium, the value of the resultant wave function equals the algebraic sum of the val- When two traveling waves having equal amplitudes and frequencies superimpose, the resultant wave has an amplitude that depends on the phase angle % between the Constructive interference occurs when the two waves are in phase, corre- sponding to % " 0, 2&, 4&, . . . rad. Destructive interference occurs when the two waves are 180° out of phase, corresponding to % " &, 3&, 5&, . . . rad. Standing waves are formed from the superposition of two sinusoidal waves having the same frequency, amplitude, and wavelength but traveling in opposite directions. y " (2A sin kx) cos $t (18.3) Hence, the amplitude of the standing wave is 2A, and the amplitude of the simple har- nodes) occur at x " n'/2 (n " 0, 1, 2, 3, . . .). The maximum amplitude points (called antinodes) occur at x " n'/4 (n " 1, 3, 5, . . .). Adjacent antinodes are separated by a distance '/2. Adjacent nodes also are The natural frequencies of vibration of a taut string of length L and fixed at both ends are quantized and are given by n " 1, 2, 3, . . . (18.8) where T is the tension in the string and + is its linear mass density. The natural fre- 1 , 2f 1 , 3f 1 , . . . form a harmonic series. An oscillating system is in resonance with some driving force whenever the fre- quency of the driving force matches one of the natural frequencies of the system. When Standing waves can be produced in a column of air inside a pipe. If the pipe is open at both ends, all harmonics are present and the natural frequencies of oscillation are n " 1, 2, 3, . . . (18.11) If the pipe is open at one end and closed at the other, only the odd harmonics are n " 1, 3, 5, . . . (18.12) The phenomenon of beating is the periodic variation in intensity at a given point due to the superposition of two waves having slightly different frequencies. f n " n v 4L f n " n v 2L f n " n 2L √ T S U M M A R Y Take a practice test for this chapter by clicking on Does the phenomenon of wave interference apply only to 2. As oppositely moving pulses of the same shape (one up- ward, one downward) on a string pass through each other, 1. 3. Can two pulses traveling in opposite directions on the same string reflect from each other? Explain. When two waves interfere, can the amplitude of the resul- 5. For certain positions of the movable section shown in Fig- ure 18.5, no sound is detected at the receiver—a situation 4. Q U E S T I O N S |