Three force $F_1=10 \mathrm{~N}, F_2=8 \mathrm{~N}, F_3=6 \mathrm{~N}$ are acting on a particle of mass 5 kg . The forces $F_2$ and $F_3$ are applied perpendicular so that particle remains at rest. If the force $F_1$ is removed, then the acceleration of the particle is: