$\overrightarrow{A}$ and $\overrightarrow{B}$ are two non-zero vectors inclined at an angle $\theta$, $\hat{\mathrm{a}}$ and $\hat{\mathrm{b}}$ are unit vectors along $\overrightarrow{A}$ and $\overrightarrow{B}$ respectively. The component of $\overrightarrow{A}$ in the direction of $\overrightarrow{B}$ is

Figure shows three forces ${\overrightarrow{F}}_1,{\overrightarrow{F}}_2$ and ${\overrightarrow{F}}_3$ acting along the sides of an equilateral triangle. If the total torque acting at point $O$ (centre of the triangle) is zero then the magnitude of ${\overrightarrow{F}}_3$ is