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Playing the 'random floor' game

  1. Dec 6, 2004 #1

    Alkatran

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    You have a block of wood submerged in a butcket of water. The block of wood is connected to the bottom of the bucket by a spring. The block of wood is stationary (the spring is not oscillating it).

    Now you place this system in an elevator and start playing the 'random floor' game.

    When the elevator is accelerating upwards, does the block move up or down relative to the bucket?
    Ditto for accelerating downward.
     
  2. jcsd
  3. Dec 6, 2004 #2
    When the acceleration is down:
    The net gravity & force on both the wood and the water would decrease, and their “weight”. The difference between the weight of the wood and the displaced water would decrease lowering the force on the spring from the wood.
    The spring would pull the wood deeper. "Down"

    When the acceleration is up:
    The net gravity & force on both the wood and the water would increase, and their “weight”. The difference between the weight of the wood and the displaced water would increase raising the force on the spring from the wood.
    The spring would be pulled longer by the wood till it floated. "up"

    Randall B
     
  4. Dec 6, 2004 #3

    Alkatran

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    That's right!

    Maybe another physiquee one:

    You have a wheel with a short rod through the axle. You attach one end of the rod to a string attached to some fixed point and hold the other end up.

    You get the wheel spinning very quickly and release your end, what happens?

    Code (Text):

          _
        | | String holding one end of axle
    You-|-o
        |   spinning wheel
     
     
  5. Dec 6, 2004 #4

    Gokul43201

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    Initially, the wheel will remain fairly horizontal and precess about the fixed point on the ceiling, maintaining its inclination. Eventually, it will decay and drop slowly downwards, as the angular velocity of the wheel about the axle dies.
     
    Last edited: Dec 6, 2004
  6. Dec 6, 2004 #5

    BobG

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    Which direction did you spin the wheel? Ah, heck, I can answer both ways:


    If you were a fly on the axle of the wheel where it hung from the string looking out towards the wheel and the wheel looked like it was spinning clockwise, the wheel will 'hang' with the axle perpendicular to the string, but rotate clockwise about the string (as you were looking down).

    Spin the wheel the other way, the axle rotates counter-clockwise about the string.
     
  7. Dec 9, 2004 #6
    Well the first thing would be that the tie point to the axle would swing away from me shuch that the center of the Wheel would line up under the tie point above with the axle remaining flat near perpendicular to the string.

    To maintain this position the energy in the spinning wheel must be drawn out of the wheel to replace the force from my finger that had been holding that end of the axle up.
    Assuming friction is very small the force and work of holding the axle up will cause a more rapid decay of the spin as energy is used to hold the axle up.
    (More rapid than if two strings were used to support both ends of the axle)
    Some of the energy will also go into rotating the axle around the string as the decay progresses and the flat angle starts to drop.

    Randall B
     
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