- #1
Kavorka
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Homework Statement
Suppose that the mass in a mass-spring-dashpot system with m = 25, c = 10, and k = 226 is set in motion with x(0) = 2 and x'(0) = 4. Find the smallest time t>0 for which x(t) = 0
Homework Equations
mx'' + cx' + kx = 0
The Attempt at a Solution
I rushed through this a bit due to lack of time today so there may be mistakes, I just need to be sure I'm doing this correctly.
The differential equation is: 25x'' + 10x' + 226x = 0
Which has the characteristic equation: 25r2 + 10r + 226 = 0
with roots: r = -(1/5) +/- 3i
Giving the general solution:
x(t) = e-(1/5)t(Acos(3t) + Bsin(3t))
Plugging in the initial conditions:
x(0) = 2 = A
x'(t) = -(1/5)Ae-(1/5)tcos(3t) -3A e-(1/5)tsin(3t) - (1/5)e-(1/5)tsin(3t) + 3Be-(1/5)tcos(3t)
x'(0) = 4 = -(1/5)(2) + 3B
B = 22/15
Finding combined solution:
x(t) = Ce-(1/5)tcos(3t-a)
C = (A2 + B2)1/2 = 3(346)1/2/15
a = tan-1(B/A) = 0.6327 radians
x(t) = (3(346)1/2/15)e-(1/5)tcos(3t - 0.6327)
If this is right (it seems too messy, might have messed up), how would I solve it when equal to zero?