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Totally random?

  1. Apr 4, 2008 #1
    I am reposting this post because i forgot I had already posted a topic with the same subject, I study physics at uni, but spend my weekends down the casino on the roulette table. I tried to explain to my friend that the reason roulette works is that the centrepetal force of the ball is great than the force exerted by gravity, therefore the ball stays in the wheel. when it is less than the force exerted by gravity it will fall.

    but does the wheel spinning in the opposite direction increase the balls centrepetal force. ie if the ball stayed still and the wheel span would there be a centrepetal force on the ball. what are the equations here.

    also does all this mean that the ball will always fall out of the wheel at the same speed, because the moment the centrepetal force drops below the gravitaional force it will fall, and the centrepetal force is reliant on the speed.

    odd question i know but its irratating me

  2. jcsd
  3. Apr 4, 2008 #2
    Newton's laws of motion are only true in inertial coordinate systems (those that are not accelerated).
    Among all rotating coordinate systems with a common origin only one can be inertial. All other are accelerated, except at the origin. So rotation of the wheel has no effect on angular acceleration, only angular velocity in inertial coordinate system counts. Ground system is (aproximately) inertial system, since the rotation of the Earth is very slow.
    I have not played roulette yet, but if it works like you described, then the ball will always fall down with the same speed. But the time when this happens can be influenced by the speed of the wheel (more friction).
    Also the starting velocity is probably random enough that the outcome can't be predicted.
    Last edited: Apr 4, 2008
  4. Apr 6, 2008 #3
    If the ball is not moving in circular motion, it is not experiencing a centripetal force -- the concept does not make sense. Centripetal force is whatever force causes the accelleration necessary for centripetal motion. Without motion the question makes no sense.

    It's best to think of in terms of accelleration. If the wheel is moving under the ball but the ball is not moving, he ball is not accellerating.
  5. Apr 6, 2008 #4


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    … rolling or sliding … ? … place your bets now … !

    Hi alex! :smile:

    I think the simple answer is that the only difference which the direction of rotation can make is to add a tangential force.

    This can make no direct difference to the (radial) centripetal acceleration.

    All it could do is make the make the speed of the ball less, through friction.

    Does anyone know whether the ball on a roulette wheel rolls or slides? :smile:
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