A particle moves in the X–Y plane under the influence of a force such that its linear momentum is $ \vec P (t) = A[ \hat i cos (kt) -\hat j sin(kt)] $ where A and k are constants .The angle betweenn the force and momentum is
$ \vec P (t) = A[ \hat i cos (kt) -\hat j sin(kt)] $ $ \vec F = { d \over dt} \left ( \vec P (t) \right) $ $ = Ak \left( - \hat i sin kt - \hat j cos ky \right) $ $ \vec F . \vec P = A^2 k ( -cos kt .sin kt + sin kt . cos kt ) $ $ \vec F . \vec P = 0 $