MATLAB Answers

Inner matrix dimensions must agree

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Ben Newton
Ben Newton on 28 Oct 2019
Commented: Rena Berman on 12 Dec 2019
%ben newton swansea university 956205
k = 997.584; %stiffness, units = N/m
m = 10.692; %mass, units = kg
xo = 0.026844; %initial displacement, units = m
vo = 0:0.001:5; %initial velocity, units = m/s
t = 0.735; %time, units = s
x = 0.0852; %dispacement, units = m
c = 36.84; %damping coefficient, units = kg/s
wn = sqrt(k/m); %natural frequency, units = rad/s
ccrit = 2*sqrt(k*m); %critical damping coefficient, units = kg/s
zeta = c/ccrit; %damping ratio, no units
wd = wn*sqrt(1 - zeta^2); %damped natural frequency, units = rad/s
A = sqrt((vo+zeta*wn*xo).^2 + (xo*wd)^2/wd^2);
phi = atan((xo*wd)./vo+zeta*wn*xo); %phase angle, units = radians
x@time = A.*exp(-zeta.*wn.*t)*sin((wd.*t)+phi);
%everythings fine up until the last line, gives the error using * inner matrix dimensions must agree
all matrices used are the same shape an size etc and .* has been used
but i still get the error message
please help

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Guillaume
Guillaume on 7 Nov 2019
This question should not be deleted. This is a public forum not your personal support service. The price you pay for getting help is that your question can be used by others with the same problem.
Your question has been flagged so that Mathworks can restore it. They usually do.
Stephen Cobeldick
Stephen Cobeldick on 8 Nov 2019
Original question:
%ben newton swansea university 956205
k = 997.584; %stiffness, units = N/m
m = 10.692; %mass, units = kg
xo = 0.026844; %initial displacement, units = m
vo = 0:0.001:5; %initial velocity, units = m/s
t = 0.735; %time, units = s
x = 0.0852; %dispacement, units = m
c = 36.84; %damping coefficient, units = kg/s
wn = sqrt(k/m); %natural frequency, units = rad/s
ccrit = 2*sqrt(k*m); %critical damping coefficient, units = kg/s
zeta = c/ccrit; %damping ratio, no units
wd = wn*sqrt(1 - zeta^2); %damped natural frequency, units = rad/s
A = sqrt((vo+zeta*wn*xo).^2 + (xo*wd)^2/wd^2);
phi = atan((xo*wd)./vo+zeta*wn*xo); %phase angle, units = radians
x@time = A.*exp(-zeta.*wn.*t)*sin((wd.*t)+phi);
%everythings fine up until the last line, gives the error using * inner matrix dimensions must agree
all matrices used are the same shape an size etc and .* has been used
but i still get the error message
please help
Rena Berman
Rena Berman on 12 Dec 2019
(Answers Dev) Restored edit

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Answers (1)

Katie
Katie on 28 Oct 2019
You're missing a dot in front of the * in front of the sin
x_time = A.*exp(-zeta.*wn.*t).*sin((wd.*t)+phi);

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