Ynot, can you clear something up for me, please? I watched the little cart that could first, because for some reason the ..couldn't didn't load properly at youtube and I clicked on the other. Now, first of all, I thought that was fairly obvious that the cart would accelerate from that held position. It's being made to keep at windspeed, but should theoretically be producing enough thrust to accelerate. You let go. It does.
But, I then watched the little cart that couldn't, and it doesn't run long enough to reach steady state, or not long enough for me to judge that. I take it that it is the same setup exactly, and I also have to take it from your report that however long it ran, the cart never reached windspeed. Now, I can't for the life of me think of any other explanation than that provided by Michael, which seems to suggest that not only did the cart not beat the wind, it never really set off from being stuck (by that higher static friction) to the TT.
Now you might think of something to test that - like perhaps handicap the cart in another fashion - if you want to check. From the few seconds we get, it seems to move slightly, but does not even cross the index mark. I think you would have a strange anomaly if it had a steady state that was some fraction of windspeed when set off from a standing start, but a multiple of windspeed if set off from windspeed. The physics of that would be very odd indeed - it should have a favoured speed - above it, it will slow down - below, it will speed up. See?
So the cart that couldn't may have just been stuck like a brick on a slope, but one that, with a tiny push, would slide down the slope. It's also my experience that such an object can slide a little way, but not quite get going and stop again.
The same design can't be a cart that couldn't and a cart that could other than by some unusual effect like the static friction you have introduced into the system. This is why your idea of the cart possibly only beating the wind when held so that it gains enough thrust to accelerate was wrong, I think. It is not being held so that it is
forced to create
sufficient thrust by being held, but
prevented from
developing more by going faster. At any speed slower than that, it will be providing more thrust than is needed for the speed it's going, so it accelerates. It's being held back, and if it's being held back, it can't be the cart that couldn't.
You might be able to hold it back to something slower, and then let go. But you've already done a standing start right up to beating the wind. I think you must conclude that your idea of what it can do being increased by being held must have been wrong. One test design - maybe tricky to find the bits - would be a trailer with wheels that are turning, but have a lot of friction in the axles. Another simpler (and truer) method might be to fit a more inefficient prop, so that it goes, but doesn't beat the wind. If you hold that cart at windspeed, I'm confident it will just slow down immediately you let go.
ETA: Whoops, in all that I forgot to ask the thing I wanted you to clear up: wasn't your point that a cart that would not 'normally' beat the wind could be made to by being held at windspeed? If so, the video of the cart that could isn't any use unless it slows down again to lower than windspeed and is actually a cart that couldn't. It seems to be a cart that could, but again the video isn't very long. So, after both videos, I don't know which it is, a cart that can or one that can't. Oh yes, I do, it accelerated from windspeed, so it isn't going to slow down again in the same conditions. You seem to have a view that the forces will be different depending on how they used to be at some time in the past, which is getting dangerously close to humberism for my liking.
Keep up the good work.
