As promised in another thread: Riddle me this, riddle me that... There are two carts on the table. For each cart, there is a rope attached on one side to the cart and via a pulley it is hanging over the edge of the table. On the other end of one rope a 5 kg weight is attached and on the other rope a person pulls with a force corresponding to 5 kg. Which cart has the highest initial acceleration? a. The cart with the weight. b. The cart with the person. c. There is no difference. I think that the cart with the person pulling the rope will have the highest initial acceleration. This is due to the difference between force and thrust: the person pulling the rope immedialtly applies a thrust to the system, while gravitation does not. Any thoughts, anyone?