Drafting

None of the arguments so far is entirely convincing. In the case of the marathon swimmers the advantage to the person who is drafting off another swimmer might be explained entirely by their expending less energy without it being necessary for the lead swimmer to be expending more energy than they would to swim the same speed without someone drafting off them. The leading Olympic swimmer would move over just to avoid making their competitor faster even if the leader were not being slowed down. Conservation of momentum is not the only effect involved in drag and propulsion. An alternate way to approach the issue is to think in terms of high and low pressure. As a body glides though the water it creates an area of higher pressure in front and an area of low pressure behind, creating a net backwards force which slows it down. In order to propel ourselves forward we move our hands backward and create high pressure areas in the rear direction and low pressure in the forward direction pushing us forward. If I swim right behind someone the front part of my body is in the low pressure area behind their body so the balance of high in front of me, and low behind me, is shifted in my favour, and I have less work to do to overcome that pressure differential/drag. If the person behind is busily using their hands to pull the water directly behind my feet backwards they will be further decreasing the pressure behind me and therefore increasing the work I have to do to overcome the front-to-back pressure differential. On the other hand, if the trailing person was doing a kick set then the high pressure area in front of them could increase the pressure behind the lead swimmer and thereby actually help the person in front. Conservation of momentum plays a part in creating the pressure differentials but it isn't the only part, viscousity and wave effects are also involved. And no, I didn't think this all through before I asked! :)

None of the arguments so far is entirely convincing. In the case of the marathon swimmers the advantage to the person who is drafting off another swimmer might be explained entirely by their expending less energy without it being necessary for the lead swimmer to be expending more energy than they would to swim the same speed without someone drafting off them. The leading Olympic swimmer would move over just to avoid making their competitor faster even if the leader were not being slowed down. Conservation of momentum is not the only effect involved in drag and propulsion. An alternate way to approach the issue is to think in terms of high and low pressure. As a body glides though the water it creates an area of higher pressure in front and an area of low pressure behind, creating a net backwards force which slows it down. In order to propel ourselves forward we move our hands backward and create high pressure areas in the rear direction and low pressure in the forward direction pushing us forward. If I swim right behind someone the front part of my body is in the low pressure area behind their body so the balance of high in front of me, and low behind me, is shifted in my favour, and I have less work to do to overcome that pressure differential/drag. If the person behind is busily using their hands to pull the water directly behind my feet backwards they will be further decreasing the pressure behind me and therefore increasing the work I have to do to overcome the front-to-back pressure differential. On the other hand, if the trailing person was doing a kick set then the high pressure area in front of them could increase the pressure behind the lead swimmer and thereby actually help the person in front. Conservation of momentum plays a part in creating the pressure differentials but it isn't the only part, viscousity and wave effects are also involved. And no, I didn't think this all through before I asked! :)