Hi everybody, I apologize if something like this has been asked before but I have been unable to find an answer through searching. If a weight is carefully added to a moving cart then the cart should slow down due to conservation of momentum. How is it that the cart has accelerated without a horizontal force? This issue originally came up while discussing rotational motion. If a figure skater is spinning and brings his hands in he will speed up. How can he accelerate without any torque? There must be something very basic that I am missing here, so thank you to anybody who can help with an answer.