The discussion in the parent thread was about inertial frames of reference and whether the cart on the treadmill is a valid representation of the cart on a road. I have fixed the tags to reflect that.
It was a good idea, not least to be able to put better tags.
In the parent thread, this discussion was leading to bad tempers because it was detracting from what the majority of the participants wanted to discuss. I hope that here it can be properly debated without getting personal.
I'll try.
I specifically left cart and treadmill out of the OP for this split so as not to limit the discussion here to the one case. This thread may go it's own way.
Great.
I don't know if an earlier post of mine will arrive here from the parent thread, so I'll post it again (developed a bit). It's a thought experiment, The 1000 Mile-an-hour Treadmill....oh, and I get to The Billion Joule Bullet later.
Imagine you're in a fixed position in space w.r.t. the earth, i.e. somewhere on a line between the centre of the earth and a distant star, and directly above the equator. You therefore watch the earth rotating below you in its eastward progression. Now you focus on a part of Africa right on the equator, where there happens to be an easterly wind blowing at the incredible speed that just matches the earth's rotation. An 'easterly' of course, blows from east to west, so it is in the opposite direction of the earth's rotation, which means that the air itself stays right there under your own position, motionless with respect to you, while the earth spins under it.
Now, to anyone standing on the ground, or testing a cart there, there's an easterly blowing at the ridiculous speed of just over 1000 mph. That is because anyone living on earth has always lived their lives at whatever speed the ground is moving at. We don't even notice that we change our speed as we move between latitudes, but we do.
Normally, most of the air in the atmosphere travels with the earth's surface, more or less, which we call a 'still day', but it is not still air. What we mean is that it is still with respect to the ground. However, to our new position in space, which is obviously as still as we've ever been, the still air of a nice day would "in fact" be keeping pace with the ground, moving east at over 1000 miles per hour at the equator. I say, "in fact", because even this is a view that would only be true according to our current location in space.
Anyway, in our scenario, we're not imagining a 'still day', but that easterly wind going in the opposite direction to the earth's rotation. Now, no-one on the ground would have any doubts at all that there was a 1000 mph easterly blowing. If someone tested a cart that went directly downwind in it, they'd be happy to report that it was a genuine 1000 mph wind, and the ground wasn't moving, if they could hear themselves speak.
So what is the problem with being in a room with a treadmill moving under "still air" to test a similar cart? We need to recognise that none of these things are in fact stationary. At a temperate latitude, we can expect that the room, the air in it, the treadmill and all the land is moving east at about 500 miles an hour. Does that cause us concern? If we did the experiment outside on that ground in a 10 mph wind, are we concerned that all of it is going east at 500 mph? The speed of sound, Mach 1, is about 740 mph, incidentally.
Do we even know how fast we are rotating round the sun? It is actually about 66,600 miles per hour. That's fairly fast, but the solar system is also traveling round in the galaxy, about 447,000 mph. The group of galaxies we're in is also racing through space, and, compared to the background radiation from the Big Bang (assuming that can even be considered to give a fixed origin), we are moving at about 1.34 million mph.
Thankfully, for most of our earth-scale problems, we can just consider a nominally stationary position, and measure things from that. We do this for position, for velocity and for kinetic energy. But it is not just a helpful bit of simplification the maths allows: in the things that happen in the real world, only the relative velocities and energies are important. This is not the only consideration at larger scales or very fine measurements, but for carts and whether they can move faster than the wind, it's all we need.
It's a good job that kinetic energy is relative, not an absolute property of a body, or every bullet would impact its target with something in the region of a billion Joules, by my estimation, even if you just picked one up and threw it at someone.
Some data on how fast we're moving:
http://calgary.rasc.ca/howfast.htm