http://www2.uah.es/jmc/ai14.pdf On page 2 of the above pdf there is an example in a box with title 'Acceleration' which states that there is a difference between a man pulling on a rope with 600N force and an iron weight of 60Kg (g = 10) tied to the rope. The difference being that once the system starts moving the iron weight generates less acceleration. I don't understand the explanation given. Could someone please explain in a different way. Thanks, E.