Three blocks of masses 2 kg, 3 kg and 5 kg are connected to each other with light strings and are then placed on a smooth frictionless surface. The system is pulled with a force F from the side of the lighter mass  so that it moves with an acceleration of 1 ${\mathrm{ms}}^{-2}$.  T₁ and T₂ denote the tensions in the other strings. The value of F is