Hello, Say you have a frictionless setting. In this setting are two skateboards. One has a waterwheel (or any wheel with fins) propped up on beams so that it is on the skateboard and can freely turn. On the other skateboard is a person standing on it. If the skateboards are one behind the other (like train carts) and the person punches the wheel on a fin so that the wheel spins, the wheel will spin but neither skateboard will move. What if you change the impulse of the punch from infinitesimal amount of time to say a few seconds?