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Direction of normal reaction of the Ferriw wheel at the high

  1. Jun 26, 2015 #1
    1. The problem statement, all variables and given/known data
    As you can see , the writer state that the direction of normal reaction force(R) is acted upwards , I have another book state that the normal reaction force (R) is acted downwards , which one is correct ?

    2. Relevant equations


    3. The attempt at a solution
     

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  2. jcsd
  3. Jun 26, 2015 #2
    If the other source is describing the normal force of the seat on the passenger then it is incorrect unless the person is inverted (so that their head is toward the center). The reaction force is drawn correctly in the image you posted.
     
  4. Jun 26, 2015 #3

    jbriggs444

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    The image you attached says nothing about a "normal reaction force". It speaks of the reaction force of the basket on the person without specifying what surface it might be perpendicular to.

    The term "normal reaction force" taken at face value would mean a force which is:

    1. "normal" -- that is, perpendicular to some surface. If working in two dimensions, "normal" means perpendicular to a line.
    2. A reaction force -- that is, the third law partner of some other force.

    The force of the person on the basket would be downward and would be a "normal reaction force" because it is perpendicular to the seat and is a reaction force to the force of the seat on the person.
     
  5. Jun 26, 2015 #4

    ok , thanks for your reply . So can you explain why the direction of the normal reaction force is acted downwards when the person is inverted at the highest position ?
     
  6. Jun 26, 2015 #5
    This would be a different ride (think of a roller coaster going through a loop). A centripetal force is required to keep the passenger in circular motion. This would be the force provided by the seat and since the seat is pointing inward the force is inward.

    Of course, the ride must be moving fast enough. Suppose the ride got stuck at the top. Then the reaction force of the harness would need to prevent the person from falling in (down) and would therefore be up.

    **edited for clarity**
     
  7. Jun 26, 2015 #6
    sorry , i still dont understand why the rection frce is acted downwards in the roller coaster ride...Mind to explain further?
     
  8. Jun 26, 2015 #7
    can I explain in this way ? If the person is inverted (in roller coaster ride) the person is actually 'sticked' to the seat , there's a force to cause it to do so . So , the reaction force should be opposite to the force , to support it from falling.... So , the normal reaction force is acted downwards ( towards the centre of the wheel) * correct me if i'm wrong*
     
  9. Jun 26, 2015 #8
    If you swing something around in a vertical circle that is attached to a rope you need to pull inward. This is pull is the centripetal force that allows for the circular motion. Cut the string when the object is at the top and it will fly off horizontally; falling due to gravity.

    Any type of circular motion requires an inward force. In the case of the looping roller coaster (where the passenger's head is inward to the circle) this force is provided by the seat. The seat is above the person and there is nothing else to push inward...
     
  10. Jun 26, 2015 #9
    There's no force which causes the person to be stuck to the seat (a common misconception). Rather, the motion of the person would be in a straight line if it weren't for the seat providing the inward force.

    How does this sit with you - because the motion is circular there must be an inwardly directed force. The reaction force to the inward force of the seat on the person is the outward force of the person on the seat.
     
  11. Jun 26, 2015 #10
    So, the normal reaction force is acted towards the circle ( downnwards) to support the person.??
     
  12. Jun 26, 2015 #11
    So , the centripetal force = R (reaction force + mg ) in this case . this motion is just like roller coaster case , right ? I know what do you mean , but now , i am asking about
    do u say it wrongly ? IMO, in the roller coaster ride , the force of the person of the seat in is upward direction , since the seat is at the top . So , the reaction force is acted downwards ? Correct me if i'm wrong.
     
  13. Jun 26, 2015 #12
    In the case of the loop the loop the normal force of the seat on the person is toward the center of the circle... Whether or not it is the 'action' or reaction' is arbitrary.

    QUOTE="goldfish9776, post: 5152596, member: 561115"]do u say it wrongly ? IMO, in the roller coaster ride , the force of the person of the seat in is upward direction , since the seat is at the top . So , the reaction force is acted downwards ? Correct me if i'm wrong.[/QUOTE]

    Don't get hung up on this whole action/reaction distinction. I was saying that if you consider the inward force of the seat to be a necessary requirement for circular motion (action) then the reaction to this is the outward force of the person on the seat. Flip the action/reaction labels around if you wish.
     
  14. Jun 27, 2015 #13
    The normal force is perpendicular to the supporting surface.
    I came across this statement when I googled out some informations . In the roller coaster case , can I say that the normal recation force is acted downwards because of it is perpendicular to the supporting surface . If I consider the normal reaction force is in upward position , then the normal reaction force is in 180 degree form the supporting surface , which is contrary top the above statement ....Correct me if i 'm correct
     
  15. Jun 27, 2015 #14

    haruspex

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    In the diagram you posted, the supporting surface is a horizontal seat. The reaction force is vertical. Last time I checked, horizontal and vertical were perpendicular to each other, not at 180 degrees.
    You don't describe the situation being discussed in the 'other' book. Without that, we have no way to resolve your confusion. Brainpushups is guessing that the other book considers a rollercoaster, not a Ferris wheel, and has the passenger inverted.
    Umm... Gravity?
     
  16. Jun 27, 2015 #15
    Ya, I noticed that the person is inverted in the other book, the normal reaction is acted in downward position
     
  17. Jun 27, 2015 #16
    Indeed, that was not well phrased. The centripetal force at the top is the sum of the gravitational force and the force of the seat.
     
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