Why is my plot empty?

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Katharine Gagliano
Katharine Gagliano on 23 Apr 2022
Edited: Walter Roberson on 23 Apr 2022
%%% Solves HW #6 Q2 %%%
% ------ Defining Variables ------ %
y0 = 0;
v0 = 0;
m = 60;
k = round(mean([6,6,0,1]*1000));
b = k/20;
g = 9.81;
Wn = sqrt(k/m);
Xst = (m*g)/k;
L = (b*Wn)/2*k;
tt = linspace(0,5,101);
%%% ------ Closed Form Solution ------ %%%
%%%% BROKEN %%%%
T = tt.*Wn;
A = -Xst;
B = (L*pi*A)/(sqrt(1-L^2));
closed = @(t) exp(-L*pi*t) .* (A*cos(sqrt(1-L^2)*t) + ...
B*sin(sqrt(1-L^2)*t)) + Xst;
subplot(2,2,1)
plot(tt,closed(tt),'-')
title('Closed Form Solution')
xlabel('Time (s)')
ylabel('X(t) (cm)')

Answers (1)

DGM
DGM on 23 Apr 2022
Edited: DGM on 23 Apr 2022
The plot doesn't display because all of the output values (except for tt = 0) are NaN.
L is a relatively large number ~2E6, so for nonzero time values:
exp(-L*pi*t) % this is zero
A*cos(sqrt(1-L^2)*t) % this is -Inf
B*sin(sqrt(1-L^2)*t) % this is -Inf
so 0*Inf = NaN

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