Comment on Which is it Lemmy?
empireOfLove2@lemmy.dbzer0.com 20 hours ago
A. Object momentum relative to the world is retained thru portals regardless of the speed of the portal surface or the portal itself. The portal is only transmitting matter from A to B, it does not impart energy of any kind.
FishFace@piefed.social 16 hours ago
If the portals don’t impart energy, you wouldn’t gain speed when you put one directly above another and drop through forever. The higher portal increases your potential energy.
What if the entire room were rigged to a piston, the orange portal were held still and the room forced upwards at speed. Would you see scenario B then?
If so, how do the portals know which to choose? Remember that neither portal is actually stationary: they’re both orbiting the sun at 30km/s, etc.
empireOfLove2@lemmy.dbzer0.com 7 hours ago
Its not the portal doing that though. It’s gravity. Your own momentum moves you through the portal while gravity is acting upon you; the portal is nothing but a doorway.
FishFace@piefed.social 5 hours ago
It doesn’t matter what’s “doing” it; the energy has to come from somewhere - gravity is not a source of free energy. In the real world, you can’t build a perpetual motion machine using gravity. With portals, you could pour water through a pair of vertical portals and use it to drive a waterwheel, and the waterwheel could, say, drive a generator and power a light. It would keep going forever. The light requires energy to work - where is it coming from?
Without the portal, an object tossed down a mineshaft would accelerate until, assuming it were deep enough, it reached the centre of the earth. Then its potential energy would be zero, and it would stop accelerating - it would have reached its maximum kinetic energy and minimal potential energy, and start trading speed for height, until it reached a maximum height (at the other side of the world) and oscillated back and forth. If you tried to use this to drive a waterwheel and power a lightbulb, each time the object hit the wheel it would slow down. That reduction in speed would translate to a reduction in energy, and a reduction in maximum height, until eventually the object would stop being able to turn the wheel.
That doesn’t happen in the portal scenario, because the portal moves objects up, increasing their potential energy, no matter what.