4 ms·

>> a spinning disc in a vacuum would maintain energy indefinitely > Assuming frictionless bearings perhaps. But what does that have to do with anything? Thank

>> a spinning disc in a vacuum would maintain energy indefinitely > Assuming frictionless bearings perhaps. But what does that have to do with anything? Thanks for this, let me elaborate. First, yes, magnetically suspended. That given, take a box containing some quantity E of energy in the form of petrol or TNT, whatever. Chemical energy. Next take a box containing aforementioned disk, spinning with an energy also of E. From the outside there is no observable difference if both they are left sitting on a warehouse floor untouched, however the box with E of chemical energy is potential energy, the box with the spinning disk has kinetic energy, according to your definitions. Yet they are indistinguishable from without. That's my puzzle. (I'm being slowposted so replies may take a while)

krisoft 4y ago
> That's my puzzle. Sorry but I don't see the puzzle. Are you expecting some "different things need to be able to be distinguished when left sitting on a warehouse floor inside a box, untouched" principle? > I'm being slowposted so replies may take a while. No worries. And sorry that that happened.
- zasdffaa 4y ago
  You can't distinguish the boxes externally (just by looking anyway). Therefore you can't say which one contains kinetic energy (the box with the spinning disk) and contains the potential energy (the box with petrol in it). If you can't distinguish kinetic from potential in this circumstance, does the definition of 'kinetic' and 'potential' have any rigorous meaning? I'm fine if they don't, I'm just trying to understand.
  - CRConrad 4y ago
    > If you can't distinguish kinetic from potential in this circumstance, does the definition of 'kinetic' and 'potential' have any rigorous meaning? They're still different forms of energy even if you can't see which is which from the outside of a (non-transparent) box. Like, if you have a red ball in one and a blue ball in the other, you can't see from the outside which colour ball is in which box either. Does that mean "the concept of colours doesn't have any rigorous meaning"? No, it just means the concept of being in a non-transparent box has the rigorous meaning that you can't see what's going on inside it.
    - zasdffaa 4y ago
      I dunno. If you can't see the ball's colours you can't meaningfully say that one is red, the other blue. It doesn't mean they aren't red/blue, you just can't infer which is in which box. Same here. Energy E will hang around for unlimited time until you extract it. I dunno. Anyway, thanks.
  - krisoft 4y ago
    > You can't distinguish the boxes externally (just by looking anyway). You know what else can't be distinguished when they sit in boxes in a warehouse (just by looking)? Bricks and teddy bears. Do you find that they also lack a rigorous meaning? :)

krisoft 4y ago

> That's my puzzle. Sorry but I don't see the puzzle. Are you expecting some "different things need to be able to be distinguished when left sitting on a warehouse floor inside a box, untouched" principle? > I'm being slowposted so replies may take a while. No worries. And sorry that that happened.

zasdffaa 4y ago
You can't distinguish the boxes externally (just by looking anyway). Therefore you can't say which one contains kinetic energy (the box with the spinning disk) and contains the potential energy (the box with petrol in it). If you can't distinguish kinetic from potential in this circumstance, does the definition of 'kinetic' and 'potential' have any rigorous meaning? I'm fine if they don't, I'm just trying to understand.
- CRConrad 4y ago
  > If you can't distinguish kinetic from potential in this circumstance, does the definition of 'kinetic' and 'potential' have any rigorous meaning? They're still different forms of energy even if you can't see which is which from the outside of a (non-transparent) box. Like, if you have a red ball in one and a blue ball in the other, you can't see from the outside which colour ball is in which box either. Does that mean "the concept of colours doesn't have any rigorous meaning"? No, it just means the concept of being in a non-transparent box has the rigorous meaning that you can't see what's going on inside it.
  - zasdffaa 4y ago
    I dunno. If you can't see the ball's colours you can't meaningfully say that one is red, the other blue. It doesn't mean they aren't red/blue, you just can't infer which is in which box. Same here. Energy E will hang around for unlimited time until you extract it. I dunno. Anyway, thanks.
- krisoft 4y ago
  > You can't distinguish the boxes externally (just by looking anyway). You know what else can't be distinguished when they sit in boxes in a warehouse (just by looking)? Bricks and teddy bears. Do you find that they also lack a rigorous meaning? :)