• rog@lemmy.one
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    1 year ago

    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
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      1 year ago

      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
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        1 year ago

        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
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          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.

          • Shiki@lemmy.world
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            1 year ago

            The portal moves towards the people. It’s a hole. Momentum won’t transfer from nothing as the hole is the one moving.

              • Shiki@lemmy.world
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                1 year ago

                Because the portal is moving them through it

                Like how you would move through a hoop if it passed through you, it’s just a door through space

                • lauthals@feddit.de
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                  1 year ago

                  Do we agree that the massive block with the blue portal is not moving? So, imagine we are standing right in front of it when it happens - what happens exactly? How do the people appear?

                  • Shiki@lemmy.world
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                    1 year ago

                    Yes the blue portal isn’t moving

                    The people appear because the orange portal is moving towards them, the people then, like through a moving door, just pop out the other side.

                    It’s a door way through space, not a momentum giving portal.

                    Can you not see the orange portal moving? They don’t jump through it, this carrying their own momentum through. They are stationary through it so coming out they will also be stationary