I was thinking the other day about how conservation of momentum works in terms of a closed system consisting of a child and a swing and gravity. How does the child start moving on the swing? Does he need to push against the air to start going? Where does he extract the momentum from? If he does push against the air, does that mean that a child cannot start swinging from rest in a vacuum? This has been in the back of my mind for a couple of days, but nothing good has popped into my head.