for non-homogeneous linear differential equations L y = f y H

Transcription

1 Tues March 13: Finish Monday's notes on 5.4, Then begin 5.5: Finding y P for non-homogeneous linear differential equations (so that you can use the general solution y = y P y = y x in this section... L y = f y H to solve initial value problems). We're back to Announcements: Warm-up Exercise:

2 The motion exhibited by the solutions to the undamped oscillator DE x t = A cos x t B sin t x = is called simple harmonic motion. The reason for this name is that x t can be rewritten in "amplitudephase form" as x t = C cos t = C cos t in terms of an amplitude C and a phase angle (or in terms of a time delay ). To see why this is so, equate the two forms and see how the coefficients A, B, C and phase angle be related: must x t = A cos t B sin t = C cos t. Exercise ) Use the addition angle formula cos a b = cos a cos b sin a sin b to show that the two expressions above are equal provided A = C cos B = C sin. So if C, are given, the formulas above determine A, B. Conversely, if A, B are given then C = A B A C = cos, B C = sin determine C,. These correspondences are best remembered using a diagram in the A B plane:

3 It is important to understand the behavior of the functions A cos t B sin t = C cos t = C cos t and the standard terminology: The amplitude C is the maximum absolute value of x t. The phase angle is the radians of t on the unit circle, so that cos t evaluates to 1. The time delay is how much the graph of C cos t is shifted to the right along the t axis in order to obtain the graph of x t. Note that = angular velocity units: radians/time f = frequency = units: cycles/time T = period = units: time/cycle the geometry of simple harmonic motion simple harmonic motion time delay line - and its height is the amplitude period measured from peak to peak or between intercepts t

4 Exercise 3) A mass of kg oscillates without damping on a spring with Hooke's constant k = 18 N m. It is initially stretched 1 m from equilibrium, and released with a velocity of 3 m s. 3a) Show that the mass' motion is described by x t solving the initial value problem x 9 x = x = 1 x = 3. 3b) Solve the IVP in a, and convert x t into amplitude-phase and amplitude-time delay form. Sketch the solution, indicating amplitude, period, and time delay. Check your work with the Wolfram alpha output on the next page

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6 Case : Unforced mass-spring system with damping: (This discussion is in Monday's notes but we'll likely need the start of Tuesday to finish it.) 3 possibilities that arise when the damping coefficient c. There are three cases, depending on the roots of the characteristic polynomial: m x c x k x = rewrite as (p = c m, k = m which has roots x x c m x p x. The characteristic polynomial is r p r r = p 4 p 4 k m x = x =. = = p p. Case a) ( p, or c 4 m k ) underdamped. Complex roots r = p p = p i 1 with 1 = p, the undamped angular frequency. x t = e p t A cos 1 t B sin 1 t = e p t C cos 1 t 1. solution decays exponentially to zero, but oscillates infinitely often, with exponentially decaying pseudo-amplitude e p t C and pseudo-angular frequency 1, and pseudo-phase angle 1.

7 which has roots r p r r = p 4 p 4 = = p p. Case b) ( p, or c 4 m k ). overdamped. In this case we have two negative real roots r 1 = p p and r 1 r = p p x t = c 1 e r 1 t c e r t = e r t c 1 e r 1 solution converges to zero exponentially fast; solution passes through equilibrium location x = at most once. r t c. Case c) ( p =, or c = 4 m k ) critically damped. Double real root r1 = r = p = x t = e p t c 1 c t. c m. solution converges to zero exponentially fast, passing through x = at most once, just like in the overdamped case. The critically damped case is the transition between overdamped and underdamped:

8 Exercise 4) Classify (underdamped, overdamped, critically damped), by finding the roots of the characteristic polynomial. Write down the general solution. Could you solve the IVP for x t? 4a) x 6 x 9 x = x = 1 x = 3. 4b) x 1 x 9 x = x = 1 x = 3. 4c) x x 9 x = x = 1 x = 3.

9 The same initial displacement and velocity, and mass and spring constant - for an undamped, underdamped, overdamped, and critically damped mass-spring problem: (Courtesy Wolfram alpha).