Okay, so if I have 2 blocks of different masses 4kg and 2kg placed side by side with possibly a distance of 1 molecule between them (sorry if the diagram doesn't show that). There is negligible friction on the surface they are kept. I apply a constant force of 20N on a side of the block with mass 4kg so both move with a constant acceleration. Now I know that the acceleration would be equal to the sum of masses divided by the force applied. But here comes the problem. When I apply force on the block, it starts moving with a constant acceleration of 20/4 which is equal to 5m/s. Hence it applies a force of magnitude 5*4=20N on the second block after moving the distance of 1 molecule. So according to newton's third law of motion, the second block will apply a reaction force of 20N on the first block as long as it is in touch with it. So the two forces should cancel each other out and the first block should not move. Since the first block stopped moving, the second block should stop moving too. Then why are the blocks moving? Please solve this mathematically.
