From Mechanics

We have drawn all the forces acting on the block in the figure. The acceleration is represented by the vector \displaystyle a.

Applying Newton´s Second Law \displaystyle F=ma horisontally,

\displaystyle \begin{align} & \to :\ 40\cos {{30}^{\circ }}-F=ma \\ \end{align} ration is represented by the vector \displaystyle a.