$ \, \, \, \, \, \, \, F=\frac{dp}{dt} =-kA\, sin\, (kt) \widehat{i} - kA\, cos\, (kt)\widehat{j}$
$ \, \, \, \, \, \, \, p=A\, cos\, (kt) \widehat{i} - A\, sin\, (kt)\widehat{j}$
Since, $\hspace20mm F.p=0$
$\therefore$ Angle between F and p should be $90^\circ.$