Ok, so I was pondering something as I lie in bed...so in a system with no outside forces, momentum has to be conserved, correct? So let's say me and the earth are the system, and initially at rest, so we have zero momentum. I jump up in the air, giving myself momentum in the positive y direction, while the earth has momentum equal to mine in the negative y direction (going down, even though it's very very slight). No, once I reach the peak of my jump, I start coming down, that is, the sign of my momentum has changed. Does this mean that at that moment the earth starts to move back towards me to maintain a net momentum of zero? Also, it seems to me when I land I will be pushing the earth down even more, making the net momentum afterwards non-zero. Can anyone explain to me where my thinking is wrong?