The portal is a hole. The hole is moving. The conservation of momentum is the hole moving as it continues to move along the track. If the people start moving, where does that momentum come from?

Imagine a tennis racket with no strings. Two portals are stretched across the space the strings would normally be, back to back, one orange one blue. If you threw a ball in the air as if you were going to serve and swung the racket, the ball would pass straight through the portals as if they weren't there and would fall straight down due to gravity. The ball maintains its conservation of momentum, and the tennis racket holding the portals also maintains its conservation of momentum as it swings through the air. There is no force applied by a hole.

[–] critical@reddthat.com 8 points 1 year ago (2 children)

Lets say the tennis racket has 2 portals. One in the front and one in the back. When you swing the racket, the front portal moves forwards with some speed V. The portal on the back is moving backwards with the same speed, so -V (same speed V, but in opposite direction). A stationary ball, suspended in mid-air would have 0 speed. The racket portal approaches the ball at speed V, so the ball has a relative speed V to the racket. The portal on the back has a speed of -V and ven you combine that with the ball's speed of V, we get -V+V=0. And so the ball stays put. The portals in the image are not both in motion. The front portal is approaching the people with a speed of V and so the relative speed of the people to the portal is V. The exit portal has a speed of 0, relative to the people. When the people go through the portal, their speed is 0+V=V, meaning they get launched out the exit portal with the same speed the entrance portal hit them.

[–] rog@lemmy.one 1 points 1 year ago (1 children)

Interesting way to look at it, but I still dont see where the force is acting on the object going through the portal. The object is not in motion and will stay in that state unless something acts upon it, so where is the energy coming from to act on the object?

[–] lauthals@feddit.de 4 points 1 year ago* (last edited 1 year ago)

To make it clear from the start: I agree 100% with B - there has to be movement, because without it, people wouldn't come out of the portal at all. And if there is a movement, then the only reasonable speed would be that of the train.

But: Your question about the energy is still interesting. It must come from somewhere. And I think, the only source, from which it can come, is the train. That is, the train would lose energy and therefore slow down.