Two blocks of masses 40 kg and 20 kg are connected by a wire that has a linear mass density of 1g/m. These blocks are being pulled across horizontal frictionless floor by horizontal force F that is applied to 20 kg block. A transverse wave travels on the wave between the blocks with a speed of 400 m/s (relative to the wire). the mass of the wire is negligible compared to the mass of the blocks. the magnitude of F is