# Double derivative of a vector of variables in Simscape components.

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Roland Vet on 11 Apr 2019
Answered: J Chen on 23 Apr 2019
I'm constructing a Simscape component (.ssc file) and I'm confused by the mechanics of declaring the double derivative of a variable, specifically the double derivative of a vector of variables.
Say I declare a variable as follows:
variables
eta = {zeros(4,1),'m'};
end
Now I wish to use the double derivative in my equations can I use a variation of:
equations
eta.der.der == %...
%or
der(eta.der) == %...
end
Or must I also declare a 'derivative variable' say d_eta and use that as a intermediate value?
variables
eta = {zeros(4,1),'m'};
d_eta = {zeros(4,1),'m/s'};
end
equations
eta.der == d_eta;
d_eta.der == %...
end
PS.
Is there documentation on the working of der([...]) or [...].der?

J Chen on 23 Apr 2019
I would say you must use the second approach:
variables
eta = {zeros(4,1),'m'};
d_eta = {zeros(4,1),'m/s'};
end
equations
eta.der == d_eta;
d_eta.der == %...
end

R2018b

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