Question: Derivative of Vector (t-dependent Velocity vector, t-dependent acceleration vector)

Hi folks, I have been trying to solve that problem for the last 2 days, but I dont get it:

Lets say I have a velecity vector which is dependent from t:

v:=t->Vector(3,{1=t^2,2=3t,3=t^3})

Now I can find the velocity at any given time by typing for instance: v(4)  works perfectly.

What I want to do now is finding the acceleration vector (all components of v(t) diff t) dependent from t, so I can also find the acceleration vector at time=6 by typing: a(6)

When I try:

a:=t->Vector(3,{1=diff(v(t)[1],t),2=diff(v(t)[2],t),3=diff(v(t)[3],t)})

and type

a(t)

, I get the right result.

But, when i type

a(5)

,I get an error, because t is substittuted by 5 before executing the derivation, and a derivation by 5 is obviously not possible.

I also tried any configuration with the subs command, trying to substitute t by e.g. u, perform derivation and resubstitute u by t. Does not work. The acceleration has to be the same kind of vector as velocity (no list).

I am very greatful for your help!