Is this force a "central field" ? A partical moves in a spiral orbit given by: [tex]r=a\theta[/tex] if [tex]\theta[/tex] increases linearly with time, is the force a central field? If not, how would [tex]\theta[/tex] have to vary with time for a central force? I believe that a central force is a function only of the scalar distance, r, to the force center, and its direction is along the radius vector. I also believe that the angular momentum of a particle is constant when it is moving under the action of a central force. Even though I seem to remember the above, I'm at a loss see whether or not the above is a central force -- nor how to modify it to make it one??