I'm having trouble with the concept. I have two questions. According to various internet posts, the force of gravity of the masses attached to the Atwood machine does not act on the rope whatsoever. Is this correct? It is kind of confusing to me, because the rope is attached to these masses. So shouldn't the force of gravity on the masses effect the rope too? This means the only force acting on the rope is reactant force of Tension from the mass. But these two forces cancel each other out. If one of the masses goes up and the other goes down, this means the rope HAS TO MOVE. So, then how does the rope move? There are no forces acting on the rope, so how does it move?