1. The problem statement, all variables and given/known data a) Two astronauts, A of total mass 90kg and B of total mass 110kg are tied together by a light rope 15m long during a space walk. A starts to reel in the rope at her end. After 8.5s she has reduced the length of the rope by 5.5m. What distance does the centre of mass move in this time? Explain your answer. 2. Relevant equations x=md/(M+m) 3. The attempt at a solution a) I thought that because A was pulling the rope the centre of mass would move towards A? As the astronauts are in outer space this is an isolated system but would the pulling of the rope by A be counted as an external force? Or is it an internal force?