If you want to substitute values for q(t) and dq(t), you may fisrt substitute the value for dq(t) followed by the value for q(t). Let the value to be substituted for dq(t) be 2.
syms q(t)
f = 2*q^2;
fd = diff(f,t)
a = subs(fd,diff(q(t),t),2)
which gives an output
fd(t) = 4*q(t)*diff(q(t), t)
a(t) = 8*q(t)
Now, you may substitute the value of q(t) in a(t)
subs(a(t),q(t),5)
to get the ouput as
ans = 40
