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Unformatted text preview: Part IB Physics B Classical Dynamics and Fluids Examples 2 2009 Problem grading: (A) Problems that can be answered directly by quoting the lectured material or by straightforward calculation. (B) Problems that require some algebraic formulation and manipulation as well as calculation. (C) Problems which are either harder or longer than (B) problems. You should feel a sense of achievement in completing these. Normal Modes 1. (B) When a diatomic molecule is adsorbed onto the surface of a metal the frequency of its internal vibrational mode is changed. If we consider a horizontal surface with the axis of the molecule vertical, a simple model which might describe this phenomenon is as follows. The molecule consists of two point masses, m, separated by a light spring of spring constant k . The mass nearer to the metal is attached to a fixed point (the metal surface) with a light spring of constant K . The two springs are collinear and the motion of the masses is regarded as confined to the line of the springs. Find the normal modes and how their frequencies vary with the ratio of K to k . Sketch the results and comment on their physical significance for large and small K . [ANS: m 2 /k = 1 + 1 2 K/k q 1 + 1 4 K 2 /k 2 .] 2. (B) A uniform rod of length a hangs vertically on the end of an inelastic string of length a , the string being attached to the upper end of the rod. What are the frequencies of the normal modes of oscillation in a vertical plane? [ANS: 2 = (5 19) g/a .] 3. (C) A simple model of a jet engine comprises three identical thin rigid discs mounted equidistant on a uniform light shaft. Describe the normal modes of oscillation of the system. A small object entering the engine produces an abrupt change in the angular velocity of the first disk. Obtain an expression for the maximum resultant angle of twist of the shaft between the discs, given that the angular frequency of the lowest vibrational normal mode is . ANS: (1 + 1 / 3) / 2 - an upper bound from the difference between two non- harmonically related sine waves. 2 4. (B) M m x x 2 1 k k k (b) (a) m m x x 2 1 k K k (a) Find the frequencies of the normal modes of the two-mass system shown in (a) above, and sketch how they vary with K/k , the ratio of the spring constant of the centre spring to the outer ones. Describe how the normal modes vary with K/k . (b) Find the frequencies of the normal modes of the two-mass system shown in (b) above, and sketch how they vary with M/m (show the ratio M/m varying from 1 to ). Annotate your sketch with pictures showing the normal modes associated with each eigenvalue at M m and M m ....
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This note was uploaded on 05/09/2009 for the course DAMTP NST 1B Phy taught by Professor Sfgull during the Spring '07 term at Cambridge.
- Spring '07