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What is the direction of the Reaction force

  1. Feb 5, 2009 #1
    Hey I am confused about the direction of reaction force..

    When a ball is on a vertical track at the bottom most of the track,
    I understand that the reaction is opposite the weight of the ball, mg.

    But when it is directly above the vertical track the reaction force is towards the center, same direction as the weight of the ball.

    That means that the reaction force is not always opposing the weight?

    I think that the reaction force exists so that the object won't sink in the track..
    that is why at the bottom of the track, it points upwards..
    At above the track, it points downwards, so that the object won't sink in the track..
    Any other explanations.

    That is what I think..
  2. jcsd
  3. Feb 5, 2009 #2
    What's the geometry of this "vertical track". I'm not quite getting it. Am i alone? :confused:
  4. Feb 6, 2009 #3

    Doc Al

    User Avatar

    Staff: Mentor

    Can we assume that you are talking about a ball rolling on the inside of a vertical circular track? If so, then since the ball executes circular motion it must have an acceleration component towards the center of the circle. Which means that at the top of the motion, the net force must be downward (towards the center). That net force is the sum of the reaction force and the weight, both of which act downward.

    At the bottom of the motion, the net force must be upward (towards the center). That means that the reaction force must act upward to counteract the downward weight and provide the centripetal force.

    Correct. It's not.
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