How do we say(or prove) that when two bodies are moving together , they can be considered as one system? Suppose we have two blocks of wood kept on a friction less surface side by side and you apply a force on any one of them. I suppose from left to right, then the force equations for each block would be F - FBA -F1= MAAA FAB= MBAB And we say that both blocks will move with same accelerations.(Is this right?) F1 is the force or say resistance which block B gives to A when a Force F is applied(does this force exist?) Can we prove the accelerations equal? Or its something else?