From Mechanics

\displaystyle \begin{align} & \mathbf{a}=\frac{d\mathbf{v}}{dt}=-4\mathbf{i}-10\mathbf{j} \\ & \mathbf{F}=5(-4\mathbf{i}-10\mathbf{j})=-20\mathbf{i}-50\mathbf{j} \\ & F=\sqrt{{{20}^{2}}+{{50}^{2}}}=53\textrm{.}9\text{ N} \\ \end{align}

This is independent of \displaystyle t.