uwot wrote: ↑Tue Aug 14, 2018 8:33 pm
Noax wrote:I could compress Earth into a black hole, toss in the other planets and about 60 more solar systems worth of planets...
Let me know when you plan to do this. I'll make sure I'm down the pub.
https://xkcd.com/1680/
Noax wrote:There is actually little connection between acceleration and kinetic energy because the former is real and the latter purely abstract.
I take your point, but is that really the reason?
Seems unintuitive, no? Angular kinetic energy is quite real, so why not linear kinetic energy? There's a symmetry that seems to be missing, the way I put it.
Noax wrote:If something is accelerating, it is doing so in any frame (fixed force, not fixed A), and thus is real...
Ah, so we only feel a force if we are accelerating away from a massive object, but feel nothing if it accelerates away from us?
Presence of an imbalanced force (and the resulting acceleration from that) and ability to feel said force are two different things. You can't feel a uniformly distributed force, but barring a counter-force, you accelerate nonetheless.
Actually, forget the massive object. If the entire universe, even if empty (yeah, I know), drops from our feet, there is no force?
I don't think an empty universe dropping from our feet is distinct from it staying put. But if <everything else> does that, yes, there must be a force accounting for it. Principle of relativity doesn't say the two situations (me accelerating, or everything else accelerating) are equivalent.
What did happen to Newton's bucket?
Excellent example of angular velocity (and angular kinetic energy) being real.
Noax wrote:There is a real (frame independent) kinetic energy relation between any two objects, which is zero if they have identical velocity.
That's the epistemological nature of relativity. There is no
relation if they have identical velocity.
They are stationary relative to each other. That statement sounds like a relation to me.
Same with the Earth and Moon, neither of which need an input of energy to sustain their orbits, but if they bumped into a 'stationary' object (yeah I know), there would be a bit of a kerfuffle.
The process of the Earth acquiring its moon in the first place was itself quite a kerfuffle. I like the word. Yes, the ISS needed plenty of energy to accelerate it into its current trajectory. I spent a bit of time trying to figure out how it could be done without an increase in entropy, and it got weird, but could be done. Had to invoke chaos theory and 3-body problem to do it.
Noax wrote:My house expends no energy to counter the force of gravity, but the rocket hovering right next to it does. So not abstract, but also not directly implied by acceleration.
If you do ever shrink the world to a black hole, you will quickly find out where all that gravitational potential was being stored.
Gravitational potential is negative energy, meaning there is no limit to how far you can fall, but you can only climb so high out of your gravity well before hitting the ground state of 0 potential energy. If the earth were compressed into a black hole, it would no longer hold the house at a fixed radius. The house would fall.
But when you really get into it, the the deformation of the atoms in your foundations will increase their energy, which, on the other hand, maybe offset by the greater centripetal force in your attic.
Quite right. The house would be torn apart by the tidal forces you describe before it fell into that black hole. I'm assuming it falls straight in, which is unrealistic. In fact it has some angular momentum, and so would assume an elliptical orbit, or it would if it could stay together. It happened recently on Saturn. Some moon got too close.