i have to differentiate a*cos(alpha) wrt time. alpha is a function of time whereas a is not. If i use Dt function in-built in Mathemtica, it will automatically give assume a to be also a function of time. How to get around this problem?
I usually find it easiest to put the dependency on t into the object or not as in
In:= a Cos[alpha[t]]
Out= a Cos[alpha[t]]
In:= D[%, t]
Out= -(a Sin[alpha[t]] alpha'[t])
But you can also just define the derivative of a to be zero with respect to t before you do anything.
That helps a lot. Thanks.