The problem is a dog weighing 10lb is standing on a flatboat so that he is 20ft from the shore. He walks 8 ft on the boat toward shoare and then halts. The boat weighs 40 lb, and one can assume there is no friction between it and the water. How far is the dog from the shore at the end of this time? Alright, since there is no friction i know the center of mass between the dog and boat stays the same. So i tried setting the center of mass in the beginning equal to the center of mass at the end. However for the equation i need to know the mass of each which i have and their distance from some point(i chose the shore). Using the shore i could get the distance the dog started at 20 ft. Not sure on the rest though. Any help is appreciated.