MATLAB: Symbolic Substitution(subs) is not working , why

MATLABsubssubstitutionsymbolicsymbolic substitution

T(t) =
(25*diff(r1(t), t)^2)/12 + (25*diff(r2(t), t)^2)/8 + (25*cos(r1(t))^2*diff(r1(t), t)^2)/4 + (25*sin(r1(t))^2*diff(r1(t), t)^2)/4 + (3*(5*cos(r1(t))*diff(r1(t), t) + (5*cos(r2(t))*diff(r2(t), t))/2)^2)/2 + (3*(5*sin(r1(t))*diff(r1(t), t) + (5*sin(r2(t))*diff(r2(t), t))/2)^2)/2
T is gained from somewhere which is long story but it works perfectly fine. My code is following:
syms T r1(t) k1
subs(T,r1,k1);
P1=diff(T,k1);
Because I can't differentiate in terms of symbolic function, I wanted to replace r1 with k1, but it doesn't replace and subs function gives back these:
ans(t) =
(25*diff(r2(t), t)^2)/8 + (75*cos(r2(t))^2*diff(r2(t), t)^2)/8 + (75*sin(r2(t))^2*diff(r2(t), t)^2)/8
Somehow r1(t)'s are disappeared but couldn't been replaced with k1. Because there is not k1 in substituted function,differentiation(diff) gives back just 0(zero) which is normal.
Can someone explain me the situation please?

Best Answer

No, the substitution is working. But you have diff(r1(t),t) and when you replace r1(t) with k1, that becomes diff(k1, t) and since k1 is independent of t, the diff() becomes 0.
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