A particle moves in $x-y$ plane under the influence of a force $\vec{F}$ such that its linear momentum is $\overrightarrow{\mathrm{P}}(\mathrm{t})=\hat{\mathrm{i}} \cos (\mathrm{kt})-\hat{\mathrm{j}} \sin (\mathrm{kt})$. If k is constant, the angle between $\overrightarrow{\mathrm{F}}$ and $\overrightarrow{\mathrm{P}}$ will be :