The $40 \mathrm{kN}$ horizontal force will be resolved into force components acting perpendicular $\left(\mathrm{N}\right)$ and parallel $\left(\mathrm{V}\right)$ to section $\mathrm{a}-\mathrm{a}$.

To do this, first determine the angle $\mathrm{\theta }$ between the surface that is perpendicular to the horizontal force (i.e., the vertical plane) and the inclined plane surface $\mathrm{a}-\mathrm{a}$.