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Unformatted text preview: 18.03 PS5 Solutions—Spring 09 1a. (first solution) Given mx 00 + kx = 0, the characteristic polynomial p ( r ) = mr 2 + k has roots r = ± ( p k/m ) i . Let ω = p k/m . Then z = ce iωt is a complexvalued solution for any constant c ∈ C . The real part has the form x = A cos( ω n t φ ) with ω n = ω = p k/m Indeed, write c = Ae iθ , so that z = Ae i ( ωt + θ ) is a complexvalued solution, and x = Re z = A cos( ωt + θ ) is a realvalued solution. This is equivalent to x = A cos( ωt φ ) and φ = θ . Starting with the other complexvalued solution, z = ce iωt , yields the same family of real solutions. 1a. (second solution) We start by seeing under what conditions x = A cos( ωt φ ) is a solution to the ODE mx 00 + kx = 0: mx 00 + kx = mAω 2 cos( ωt φ ) + kA cos( ωt φ ) = A cos( ωt φ )( mω 2 + k ) which is zero exactly when ω = ± p k/m . Since frequency is positive by convention, ω = ω n = p k/m . To see that every solution is of the form x = A cos( ω n t φ ) with ω n = p k/m , one should check that the form is flexible enough to support all possible val ues of x (0) and x (0). This leads to two simultaneous equations for the two parameters A and φ . But students are not required to carry this out to confirm that there is always a solution. The fact that there are no other solutions follows from the fact that a solution is uniquely specified by the initial conditions x (0) and x (0). 1b. The initial conditions x (0) = x , x (0) = 0 for x = A cos( ω n t φ ). lead to A cos φ = x ω n A sin φ = 0 which means that φ = 0 and A = x , so x = x cos( ω n t ). The first time that x = 0, we have ω n t = π/ 2. In general, x = ω n x sin( ω n t ), so at the time when ω n t = π/ 2, then x = ω n x . The kinetic energy is (1 / 2) m ( x ) 2 = (1 / 2) mω 2 n x 2 = (1 / 2) m ( k/m ) x 2 = (1 / 2) kx 2 . 1c. Given E = (1 / 2) m ( x ) 2 + (1 / 2) kx 2 , we have E = mx x 00 + kxx = x ( mx 00 + kx ) = 0....
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This note was uploaded on 05/06/2010 for the course 18 18.03 taught by Professor Unknown during the Spring '09 term at MIT.
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