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I'm making an adventure map. In one challenge you have to shoot a minecart with a bow and arrow and destroy it in order to open a door. I have the minecart on a loop of powered rails (except for the corners, and a detector block used to determine when the minecart stops going round).

Simple experiments seem to suggest that the cart will keep moving once it's going even if I save/exit and come back, but is this always true? Is there a chance someone playing my map could get to this puzzle and find the minecart stationary?

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Well there's always a chance that something will go wrong, but I don't see how you can test it any better then completely unloading and reloading the world. Oh, and when you're done, post a link please. :-) –  John the Green Aug 13 '11 at 19:13
    
Will do! Should be ready in a few days... –  fredley Aug 13 '11 at 19:23
    
@John If you want to help me test here's the MC Forum post: minecraftforum.net/topic/552293-adv-trantorus-testers-wanted –  fredley Aug 15 '11 at 12:06
    
I don't have an account there... would that be a problem? –  John the Green Aug 16 '11 at 0:28
    
@John Catch me in chat and I'll give you the link, it's just that comments aren't the best place to handle feedback... –  fredley Aug 16 '11 at 8:58

2 Answers 2

up vote 14 down vote accepted

According to my testing, minecraft momentum is preserved when a chunk is unloaded. EthosLab noted that minecart momentum is preserved in one of his videos, but I tested it myself just to make sure.

The minecart in this screenshot is moving counter-clockwise, and it has just enough momentum to reach the Powered Rail segment once completing the loop. If I quit and then restart Minecraft when the minecart is where it is in the screenshot (just after the Powered Rails), the minecart acts exactly as it would if I hadn't quit the game (it just reaches the Powered Rails).

Minecart loop.

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I can confirm this from the source code. All "entities" (basically, things which can move… mobs, minecarts, arrows, etc.) store their speed in all three dimensions when saving. –  Ben Blank Aug 14 '11 at 5:02

I made a simple redstone clock with piston and when that location on the map was unloaded, the block had stop moving. So my guess is that the momentum is not preserved for everything that you make :S

Maybe for rails but piston with repeaters don't continue if the map is unloaded.

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Redstone clocks based on Repeaters don't continue after you reload your world, because they stay "stuck" until updated again. –  Kevin Y Aug 13 '11 at 22:17

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