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There are 3 lightbulbs behind a closed door

  1. Oct 13, 2005 #1
    There are 3 lightbulbs behind a closed door. Each lightbulb is connected to a switch outside the room. You can manipulate switches in whichever way, but once you open the door, you cannot change them.
    Match bulbs and switches.

    ps: actually i am working on it myself :tongue2:
  2. jcsd
  3. Oct 13, 2005 #2


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    Can you open the door and then close it and open it again hehe
  4. Oct 13, 2005 #3
    you wish.... :biggrin:
    my "can you" was "is there a space between the door and the floor?"

    actually i dunno, i got it off some web-site word-for-word... we'll see if someone can solve it as is ...
  5. Oct 13, 2005 #4
    So WutZ the Qustion?
  6. Oct 14, 2005 #5


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    Turn all three off for a period then:

    Switch 1: On
    Switch 2: Off
    Switch 3: Keep on for several minutes then turn it off.

    Now, open the door and report back to us what you observe! :)
  7. Oct 14, 2005 #6
    that's very clever!
    well... no, i don' t have a room just like that, but i suspect the switch3 lightbulb will have the wire inside partially lit, since not much time has passed yet.
  8. Oct 15, 2005 #7
    Let us name the switches 1,2,3. Now turn on switch 1. After, say, 2 hours turn on switch 2. And then after abou 2 mins switch off both the switches. Now open the door and feel the bulbs, the hottest one is for switch 1, the lukewarm bulb corresponds to switch 2 and the coldest one is switch 3. Voila!
  9. Oct 19, 2005 #8


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    You do have the sense of sight along with touch, you know. :biggrin:

    (Oh yeah, and there is no stipulation that all the switches need to be 'off' when observing the bulbs).
  10. Oct 19, 2005 #9
    How do we know whether a switch is on or off?

    Wouldn't we to have to assume that they are wired and assembled in the standard way?
  11. Oct 19, 2005 #10
    I think you'd have to assume that the switches flipped a certain way will correspond to the lights being on OR off. i.e. A switch flipped up should mean the light is on.

    That being said, flip switch 1 up for about an hour, while the others are down. After that, flip switch 2 up and 1 down, so that switch 2 is the only one that is up. Now open the door. The light on corresponds to switch 2. The off light that is hot corresponds to switch one, and the last, cold light is obviously switch 3s.
  12. Nov 7, 2005 #11
    so it just an assumtion ?
    is it possible to solve the puzzle wihtout making thos assumtion
  13. Nov 7, 2005 #12
    That assumption comes with the switch being outside where you can perform any test needed to determine what is on & off. Even if you use the light coming from under the door so long as no shadows under the door give a clue as to any difference between the lights. Or just use a volt meter.

    Once, you know the on/off; turn two on and the others off for at least a few minutes as shaan said.
    Then turn one off and one on – immediately enter (the one and only time you can open the door-blocking the switches) and inspect the two lights that are on quickly to see which one is hot and which is just warming up. Then check the two that are off, to see which is cold and warm.

    Oops that solved for four!
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