MAE-182A Midterm Solns W05

MAE-182A Midterm Solns W05 - Problem 1 (4+4+4+13) a....

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Unformatted text preview: Problem 1 (4+4+4+13) a. Discuss the roles of the following methods in solving linear first order differential equations including the kind of problems where these methods are applicable i) Method of variation of parameters ii) Method of reduction of order iii) Wronskian of two functions b. If y1 (t) = 1/ t, t > O is one of the fimdamental solutions of the differential equation tzy"+31y'+y=0, t>0 Find the other fundamental solution using the method of reduction of order. Also show that these solutions are fundamental solution of the differential equation. Solution Consider a linear, second order differential equation y” + p(x)y' + q(x) = g(x) . The general solution can be written as y(x) = c1 y1(x) + czy2 (x) + yp (x) where yl (x) and y2 (x) are two linearly independent or fundamental solutions of the homogeneous equation y” + p(x)y' + q(x) = 0 while yp (x) is any solution or particular solution of the non- homogeneous equation y" + p(x) y’ + q(x) = g(x) . Various methods are available to find the solutions yl (x), y2 (x) and yp (x) i) Method of Variation of Parameters Application: This method is used to find y p (x) When both fundamental solutions y1(x) and y2 (x) are known. Method: We write the particular solution yp(x) as yp(x) =ul(x)y1(x)+u2(x)y2(x) where u1 (x) and 2.42 (x) are two unknown functions of x . These functions are determined from the condition that yp (x) is a solution of the non-homogeneous equation. Substituting in the differential equation, we can show that WP}! y,2(t)g(t)dt V, , 1420):"! x1(t)g(t)dt I y1(t)y2(t)"y2(t)y1(t) y1(t)y2(t)_y2(t)y1(t) Limitation: None as long as the integrals exist Method of Reduction of Order Application: This method is used to find the other fundamental solution y2 (x) when one of the fundamental solution y2 (x) is known. Method: We write y2 (x) as y2 (x) = v(x) y1 (x) .Substituting in the homogeneous differential equation, we can show that x —]p(t)dt DOC) = I e ds (y1(s))2 Limitation: None as long as the integrals exist Wronskian of Two Functions Application: This method is used to test the linear independence of two fimdamental solutions y1 (x) and y2 (x). This test ensures that two solutions of the homogeneous equations can be used as fimdamental solutions Method: We define the Wronskian W[ y1 (x), y2 (x)] = W(x) of two given fimctions as W(x) = y1(x)y;(x)- y2(x)y1'(x) . For y1(x) and y2(x) to constitute a set of fundamental solutions, W(x) ¢ 0 within the domain of existence of the differential equation. Limitation: None (b) In this case y1 (t) =% and p(t) = Hence t -]p(x)dxd 1 t y2(t) = y1(t) e—z—s = i, t > 0 {y1(s)} t In this case the Wronskian W(t) = y1 (t) 5’2 (t) — y1 (t) 5’2 (t) = Z13— ¢ 0, for t > 0. Hence yl (t) and y2 (t) are fundamental solutions. Problem 2 (5+10+5) Consider the displacement u(t) of dynamic system defined by mass m , spring constant k and damping coefficient c , and governed by the differential equation mii+c11+ku=F(t)=E,sin(wt), m,c,k>0 a) Define the steady state response of the system. 2 b) Show that when G 2km when the fiequency a) of the external force F (t) is equal to a) 1—62 w-JZ 0V 2km, 0 m c) In an experimental design, it is desired to have a maximum steady state amplitude at a driving frequency of 31—5- times the natural frequency (00. If the spring is <1, the amplitude of the steady state response is maximum stretched 10 cm by a force of 3 newtons, and m = 2 kg , find the damping coefficient c. Solution: 3‘ ’ {é<’nt.,¢ (I sagging“. a; +9“ ‘.lf,a.(’,M-f.-,,i Iqaai‘m“ QM bl OHM!» 5/) u(t): “((é’4. vac. “0(‘3‘ sd‘hfi '{LA Wremfgmq -€,qwfi§tirv\/ M - FM" ' cw“ Jay“; M 4.10%.; “w a A or: .u~ W 3 “if “i t win chm M M.(:K70a o u".d )vai-Kd Zuvt‘” 'm I v- A”. 0;“ it v. .4 M 4L} ' *"wv‘v $4“; § 44;!“ 1 ‘6 ' M 9 i .14» r {M in» (W a} a A-EW; Arkwr- akfil ': i . ' 'lr I: m it? ( l, (k, “AM “it... whiz/C fl at" J ‘ mm w ‘ $w~“‘3 t-hri- harm.— Ls“ Mk- rA 4k- [r‘f‘kwu a. ‘ A.“ A ‘ bum-j 4).; (A? f .1 .,A;" W3 ('awt‘ : “LI/9"“ " ‘n 4"“ :1. -' (cum (9 1": kit/9 9W m Sh x. “12m WW Fm“, {2,540.30 , M is “+la;‘§ Uku’) ; FE:- (Uk '8' M ‘ " “‘w.t_ux) b .... 93o: RM 1r 1.. L 1. 'L ‘v 1...) a. I a 5 w (N°,u)vrlo .- Solution to Problem 2: Continued For My: WW 3331“ [’51 t L 9"? C. 7JV‘W‘ LI‘M 2/ Bv-J: k: 1”” ,,3g, ’ 4.,qu v.0». 60'“ a” in!) Problem 3 (5+5+8+4+3) 1. What are the characteristic differences in the solutions of a linear, second order differential equation (LSDE) near an ordinary point and a regular singular point? 2. Consider the power series representation of two fundamental solutions y1 (x) and y2 (x) near a regular singular point x = x0 of a LSDE. Which one of the following statements is valid and why? Explain your answer. 2a. lim yl (x) and lim y2 (x) are always finite x—-)xo 2b. lim y1 (x) and lim y2 (x) may not be finite x—~)xo 3a. Derive the general solution of the Euler equation for x > 0 xzy”+2xy'—6y = 0 3b. Using the general solution, derive the complete solution that satisfies the boundary conditions y(1) = l, y'(l) = —1 Is the solution bounded near x=0? 3c. The displacement y(x) of a physical problem is governed by the Euler equation in 3a. If the solution near x = 0 is bounded, write down the general solution of the displacement y(x). \- Nrtw a.) OVJH'W P3»? burr)!» ()0 '36s) :2 QM (x’x.)"‘ 59M fl)». 16-8 : 4.; 6 [SIM “:0 yea Mm U «A3,».Ja/«f ’nmfir‘lkr Fahd: V;yo 06 C*’y°)r Z ah()‘")‘o)‘ ray-G “MM 1,; M f 1: ~ " .— ‘K-ax. a i, o v " ‘Fthc-h .4, r70 Maud‘J 4 "4° MM WWW ‘55. 34. Solution to Problem 3: Continued .... .. “we: r $-Knéfiw T(r-!’) 4. 1y- — a, a, a or (v—L)(v?3) -: c V‘9IIVLs—3 (“Man-U“ all—Mtg saw}: ‘éfi-)-; Cox-f 6.x , X)? —‘r %’(¥3': ZCOXA—‘aC‘X ‘30)1‘f,3’(0=~1 .5.) <0 :3”— , c, ;; “‘“““ ‘36!) aixm-rlxp )(70 5' 5 145m at“) a 00 3"‘9u I( “a s.&d;.w« 8a (Gouda M x—go , {M g; o , Ham“ I‘ve 36‘)“; «Cox’- Problem 4 (10+10+5) a. Find the recurrence relation between the coefficients of the power series solutions of the following differential equation (1+x2)y'—4xy'+6y=0 aboutx= 0 b. Using the recurrence relation, find the power series expansions of two fundamental solutions of the differential equation about x=0. c. What are the radii of convergence of the fimdamental solutions. 40‘ x20 ‘3“‘~ W‘:K&Y3 Vazvv‘ fl 4kg C.(:G¢flm’-"/Q' Herc; M ?‘wtx AMen 3&QM'4-Wr (an... (3' «’71-?MA7A'J M 00 ‘3")? E61ka 3: any“ , 5kamfl‘h‘dv '6‘»! Swrlr‘ar Mac» ‘ (wwflmh QMKC‘ , Wk, q.“ 7,9 , n 4. Sw‘osbhku‘ .3— JL‘, kgnzé’m! (quip... w-t m-z. _ 0*k"k‘(n”')a“nx Athfihxfi‘+é4h¥“ :0 (7 yr x (“+l)ah+l+ 1 ‘9 lie“, “aw-1 t "' 64—)6'3) a.“ “*2 h+l) 4" QM': 9) “‘0 “3° 6 q2. 5’5xo “’3‘ --) 13 C: n-fsm' mama M=a'_, awaawg “.7,” a,“ N53“ @519 ———h CL7ZCL‘1-éauaafl :0 J: .. [Who ’ &°(t_3)"p) ‘_ a." X3 FWIMW S. 3 9C3 LL a‘(;)‘:: \a-BV , 3 1a,. . 0+ WWMV6‘MC‘ : w ...
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This note was uploaded on 11/02/2008 for the course MAE 182A taught by Professor King during the Spring '08 term at UCLA.

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MAE-182A Midterm Solns W05 - Problem 1 (4+4+4+13) a....

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