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string. The ends of the string, because they are fixed, must necessarily have zero normal modes, each of which has a characteristic frequency that is easily calculated. This situation in quantization. Quan- tization is a common occurrence when waves are subject to boundary conditions and Figure 18.11 shows one of the normal modes of oscillation of a string fixed at both ends. Except for the nodes, which are always stationary, all elements of the S E C T I O N 18 . 3 • Standing Waves in a String Fixed at Both Ends 553 L (a) (c) (b) (d) n = 2 n = 3 L = λ 2 L = – λ 3 3 2 n = 1 L = –
λ 1 1 2 f 1 f 3 f 2 N A N λ λ λ Active Figure 18.10 (a) A string of length L fixed at both ends. The normal modes of vibration form a harmonic series: (b) the fundamental, or first harmonic; (c) the sec- ond harmonic; (d) the third harmonic. At the Active Figures link at http://www.pse6.com, you can choose the mode number and see the corresponding standing wave. N N N t = 0 (a) (b) t = T/ 8 t = T/4 (c) t = 3T/ 8 (d) (e) t = T/ 2 Figure 18.11 A standing-wave pattern in a taut string. The five “snapshots” were taken at intervals of one eighth of the period. (a) At t " 0, the string is momentarily at rest. (b) At t " T/8, the string is in motion, as indicated by the red arrows, and dif- ferent parts of the string move in different directions with dif- ferent speeds. (c) At t " T/4, the string is moving but horizon- tal (undeformed). (d) The motion continues as indicated. (e) At t " T/2, the string is again momentarily at rest, but the crests and troughs of (a) are reversed. The cycle continues until ultimately, when a time interval equal to T has passed, the configuration shown in (a) is repeated. |