tailieunhanh - Advanced Engineering Mathematics 2011 Part 10

Tham khảo tài liệu 'advanced engineering mathematics 2011 part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | The Wave Equation 349 where f 1. Show that the solution in this case is . . 4ui 1 nitex . mca . nirxx . inirct u x t -rir- sin - sin sin -1 sin - 7TJC z n2 V L L L L n l As in part a the nth mode is absent if the origin is at a node. The intensity of the overtones are now of the same order of magnitude higher harmonics overtones are relatively more in evidence than in part a . c Bowed violin string The theory of the vibration of a string when excited by bowing is poorly understood. The bow drags the string for a time until the string springs back. After awhile the process repeats. It can be shown9 that the proper initial conditions are u x 0 0 0 X L and ut x 0 4 c L x L2 0 X L where 0 is the maximum displacement. Show that the solution is now 9 See Lamb H. 1960 The Dynamical Theory of Sound. Dover Publishers Mineola NY Section 27. 350 Advanced Engineering Mathematics D ALEMBERT S FORMULA In the previous section we sought solutions to the homogeneous wave equation in the form of a product X x T t . For the one-dimensional wave equation there is a more general method for constructing the solution. D Alembert10 derived it in 1747. Let us determine a solution to the homogeneous wave equation d2u _ 2a2u dt2 c dx2 oo X oo 0 t which satisfies the initial conditions X J 0 u x 0 r and g x oo X oo. We begin by introducing two new variables ị TỊ defined by x ct and TỊ X ct and set u x t w 7 . The variables and T are called the characteristics of the wave equation. Using the chain rule d _ dị d dr d _ d d dx dx dị dx dr d dr d d d dr d d d dt dtd dtdrj cdị cdĩ 4 4 d2 _ di d i d d dr d i d d dx2 dx d d di dx dr dr d2 a2 a2 AC2 a 2 d dr dr 2 and similarly c2 c2 Ĩ_2X- Ắ at2 a 2 d dr dT 2J so that the wave equation becomes Sr o. d dr The general solution of is w e 7 F O ơ 7 . 10 D Alembert J. 1747 Recherches sur la courbe que forme une corde tendue mise en vibration. Hist. Acad. R. Sci. Belles Lett. Berlin 214-219. The Wave .

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