From Mechanics

We first obtain the acceleration \displaystyle a.

Using the kinematic equation (see section 6)

\displaystyle {{v}^{\ 2}}={{u}^{\ 2}}+2as

gives

\displaystyle \begin{align} & {{0}^{2}}={{50}^{2}}+2\times a\times 300 \\ & a=-\frac{2500}{600}=-4\textrm{.}167\ \text{m}{{\text{s}}^{-2}} \\ \end{align}

Then \displaystyle F=ma\ gives

\displaystyle F=1000\times 4\textrm{.}167=4167\ \text{N}