Yeah, this one took me a while to wrap my head around and intuitively “get it”. I first learned it was true from that mythbusters episode where they correct their past mistakes… and even they had thought that two cars hitting head on would receive the same energy as hitting a stationary wall at the speed of the sum of their speeds. They were corrected in letters written to them, and then they experimentally verified it.
And even seeing the experimental verification, it still took me a while to really get it. The opposite speeds cancel out, making you go from your speed to zero. Same as if you hit a brick wall at that speed.
Let’s say the two cars are going 50 mph (kph, whatever unit you want). 50-50=0. You experience the same as hitting the brick wall. It’s the difference between initial speed and final speed that matters, not the sum of their speeds.
I don’t think your premise is correct. From memory, and scanning a couple episodes illegally uploaded to YouTube, it doesn’t look like Captain Planet flies away at the end. Usually, the scene just transitions to later after he’s gone, but the few times we see it, he dissolves into light, presumably returning the rings powers to the planeteers and disincorporating. He doesn’t go anywhere, he bodily ceases to be.
Where does his mind go? I think he’s a spirit. Where does team spirit go after the football game is over and everyone goes to bed? It still exists, but it’s not… here. It has to be activated to be tangible.