I'm not sure if the problem makes any sense, how would you compute the gradient of cos(wt) ? What are the variables?
Normally "grad" is applied to a function of 2 or 3 variables: if f is a function of x, y, and z, f(x,y,z) then . The div of a vector function is the scalar function . So div(grad(f)) would be .
With a single variable like t, one way to interpret this is with that single variable replacing one of the x, y, z variables, the others 0. In that case, we would have "div(grad(f))" equal to , just the second derivative. Another way to interpret this is as a one component of a two or three component vector. But then I don't know how you would apply "grad" to a vector valued function. Perhaps it would help if you posted the entire problem.