Wow. I wrote a post about Directly Down Wind Faster Than The Wind (DWFTTW) vehicles. At the time of this post, there are 37 comments for the original post. That is way more than I expected. This is very popular topic. Clearly, it brings a lot attention to Dot Physics. The first question – is this the kind of attention a naked person running through campus gets or is it a different kind of attention? I really don’t mind either way.
I am going to try to look through all the posted material and see if I can add or subtract anything for the discussion.
First – the parameters of the discussion
The discussion should be about directly down wind faster than the wind vehicles. Yes, an ice sail boat goes faster than the wind. I think sail boats can do this also. However, they go this speed when they are NOT going in the same direction as the wind but perpendicular to it or something. This discussion is for a vehicle that is going say 15 mph in the same direction as a wind that is maybe 8 mph.
Ok – let the discussion begin.
In my original post, I said that I would ignore friction between the wheels and the ground. If indeed there can be a DWFTTW vehicle, it would likely need friction. My argument was based on vehicles that used things like jet engines.
Work and Energy
Let me start off by re-emphasizing some fundamental principles. In particular, the work-energy principle. This says that the total work done on a system is equal to its change in energy. Let me take the vehicle as the system (but not the air). Also, let me assume that it is moving at a constant speed faster than the wind (directly down wind). Here is a diagram:
If the vehicle is traveling at a constant speed, the net force on it must be zero. In this case, I included everything acting on the vehicle (which I model as just a point so that friction can make it change momentum and it doesn’t matter where the forces are applied). Note that there is the gravitational force and the force from the ground. These two forces are the same magnitude but opposite directions. Thus, they do not change the vertical momentum of the vehicle. Also, since these forces are perpendicular to the direction of travel, they do no work. Really, I could leave these out, but I want to be complete.
Now for the important forces. The force from the air and the force from the ground in the direction of motion (friction). Did I leave out any forces? I don’t think so. In terms of work-energy, the air does negative work on the vehicle (because it is in the opposite direction as the motion). Friction does positive work. Where does the energy come from for friction to do positive work? That is the key question. If this were an automobile, that energy would come from burning gasoline. In the DWFTTW, I am not sure where this energy comes from. Not saying it doesn’t really work, but for me – this is the key question.
Moving against the wind
One point that was brought up was that boats can said into the wind (in a manner). From what I understand (which clearly isn’t everything) this has a lot to do with aerodynamic lift from the sails. Anyway, if a boat can do it, why not a DWFTTW vehicle? But remember, this is a discussion about faster than the wind, not against the wind.
I bring up moving against the wind because I can think of one case that might work moving against the wind (and I think this was mentioned in the forum). Suppose I had a wind generator that charged a battery or capacitor. After a while, the generator should be able to build up enough energy to get the vehicle moving some. However, this would not be a constant self-sustaining vehicle. It would need to stop and recharge. Also, it would not be moving faster than the wind, but against the wind.
Other stuff from the comments
Chris posted this http://home.san.rr.com/tadhurst/DWFTTW.htm. It seems like a discussion about the forces on the prop.
JB posted this link http://www.nextenergynews.com/news08/next-energy-news8.29.08b.html. Clearly there is some evidence of wind powered cars that can go directly into the wind. But the article does not say anything about going down wind faster than the wind
Jason posted this link to the Brennan torpedo http://en.wikipedia.org/wiki/Brennan_torpedo. I don’t think it really applies to this situation since it clearly has an external power source.
Is there a conclusion? I am still not clear. If you have a car moving faster than the wind, into the wind, powered by the wind – where is the energy coming from? If it is moving with the wind and then reaches the wind speed and then moves faster than the wind, how does it make this transition? There seems to be a lot of stuff out there that says this can happen, but you can’t get something for nothing. I am sure I will get just as many comments on this saying all sorts of things. I really appreciate all the feed back and the civil discussions that have followed.