A system of three particles having masses ${m_1} = 1\,kg{\mkern 1mu}, \,\,{m_2} = 2\,kg{\mkern 1mu}\,{\rm{and}}{\mkern 1mu} \,\,{m_3} = 4$ kg respectively is connected by two light springs. The acceleration of the three particles at any instant are $1{\rm{m}}{{\rm{s}}^{ - 2}},2{\rm{m}}{{\rm{s}}^{ - 2}}\,and\,0.5{\rm{m}}{{\rm{s}}^{ - 2}}$ respectively directed as shown in the figure. The net external force acting on the system is