Two blocks of masses m_1=8 ~kg and m_2=7 ~kg are connected by a...

Two blocks of masses $m_1=8 \mathrm{~kg}$ and $m_2=7 \mathrm{~kg}$ are connected by a light string passing over a light frictionless pulley. The mass $m_1$ is at rest on the inclined plane and mass $m_2$ hangs vertically. The angle of inclination is $30^{{}^{{}^{\circ}}}$. Therefore, the force of friction acting on $m_1$ is