The Problem: A 4 kg dog stands on a 16 kg flatboat at distance 12 m from the shore. The dog walks 5 m along the boat toward the shore and then stops. Assuming no friction between the boat and the water, find how far the dog is then from the shore. The dog's displacement is towards the shore. The boat's displacement is away from the shore. My work: I set the shore as the origin. Then, ((4 kg x 12 m) + (16 kg x 12 m))/(4 kg + 16 kg) = 12 m for original center of mass. ((4 kg x 7 m) + (16 kg x 12 m))/(4 kg + 16 kg) = 11 m. So I get a 1 m difference. My question: does it mean that the dog is currently 7 m + 1 m away from the shore, or does it mean that the dog is 12 m - 1 m away from the shore?