Previous threads on the subject have gone badly and/or have moved quickly and gone round in circles without really getting anywhere. The question is whether a cart with a propeller powered by the wheels (or vice versa) can move downwind faster than the wind. It has (supposedly) been demonstrated on the ground and on a treadmill. The debate has taken two directions, first whether the two scenarios are identical and second, whether they are really showing what they appear to show (whether they are hoaxed). The scenario is actually relatively simple, but it is counterintuitive and easy to misunderstand (I did when I first thought about it). The only way to get it straight is to draw a diagram and make sure it is clear to everyone what is happening. And the only way to ensure that people keep focused is for me to control the thread. So that's what's going to happen. If you want to post, PM me and I'll post applicable parts here. No direct posting will be allowed. Now I will be away for a few days around Thanksgiving and may not have computer access and I don't have access to PF at work. So we'll just have to see how it goes. So here goes.... I've started with a diagram, similar to one I posted in the last thread. I have gotten a few responses that tell me that I need to back up a step, so this one is not exactly the same as the one I posted before. Describing it in words: The cart is sitting on a surface and we are working from the frame of reference of the cart (as if you were sitting in it). What the surface is is irrelevant at this point. The surface is moving from the right to the left, past the cart. The cart's wheels and propeller are spinning because they are geared together. The cart feels no wind from any external source. An important things to note, as it has been misunderstood previously: *The cart does not, in either scenario, start off stationary wrt the surface. When the scenarios start, the wheels are spinning and the cart and surface moving wrt each other because in either case (whether on the ground or on the treadmill) a person pushes it up to speed.* Does everyone understand and accept this as a starting point for the problem?  Heh - made a mistake in the diagram. Fixed at 5:55 pm EST.