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Direction of reaction force in a circular motion

  1. Jul 1, 2015 #1
    1. The problem statement, all variables and given/known data
    Thewheel is rotated in the anticlockwise direction...The correct reaction force is drawn using the blue pen . I don't understand why the reaction force isn't directed to the left ? which is drawn using the red colour ?
    IMO, the red colour should be correct because the supporting surface is on the right , ( the person sticked to the right)

    2. Relevant equations


    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Jul 1, 2015 #2

    andrevdh

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    You are probably thinking of the resultant force which should be
    to the left as indicated in red. The reaction force is from the floor which
    is the friction and normal force combined which would point
    upwards at an angle as indicated by the blue N3. The red force
    is the blue N3 and W combined - the resultant of the forces.
     
  4. Jul 1, 2015 #3
    So, which is correct ?
    Centripetal force is the resultant force of R3 and W (mg ) ?
    Or R3 is the resultant force of centripetal acceleration and W(mg ) ??

    P/s : I know the red line should be centripetal acceleration force now .
     
  5. Jul 1, 2015 #4

    andrevdh

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    According to N2

    a = Fresultant/m
     
  6. Jul 1, 2015 #5
    what is the relationship of it and the question??
     
  7. Jul 1, 2015 #6
    Can you explain this statement in more simple sentence??
     
  8. Jul 1, 2015 #7
    which is the resultant force ???
    the red line or the blue N3?
     
  9. Jul 1, 2015 #8

    andrevdh

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    The clue for me was "reaction force".
    That is the person is pushing "down" on the floor due to being spun around.
    The floor reacts (reaction force), your R3, by pushing back.
    We can think of this reaction force as two forces combined, the normal force,
    perpendicular to the surface, and friction, parallel to the surface.
     
  10. Jul 1, 2015 #9

    andrevdh

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    According to Newton's 2nd law a body accelerates in the direction of the resultant force
    as I tried to show in one of my previous posts.
    According to this what should the resultant force then be? Red or blue?
     
  11. Jul 1, 2015 #10
    I can understand post # 8.
    the resultant force should be the red line. but according to phytagoras's theorem , the R3 looks like resultant force , since W and Fc is perpendicular to each other ( as shown in the triangle drawn) which is making me confused now.
     
  12. Jul 1, 2015 #11

    andrevdh

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    It is just a question which forces the object is experiencing,
    in this case W and blue N3. The resultant is the (vector)
    sum of these. That means we can replace W and blue N3
    with the red N3 in the diagram.
     
  13. Jul 2, 2015 #12
    so , it has completely nothing to do with the hypotenuse of triangle (maths concept ) ?
    The resultant force can be in any direction depending on the question although it does not comply with hypotenuse of triangle (maths concept )??
     
  14. Jul 2, 2015 #13

    CWatters

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    The resultant force is the vector sum of all the forces acting on the body. The vector sum of two forces only equals the hypotenuse of a triangle if the two forces are at right angles (orthogonal).

    Your diagram does not appear to show _all_ the forces acting on the body. I understand W = Weight. Where is the centripetal force shown?
     
  15. Jul 2, 2015 #14

    andrevdh

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    As I understand it the person is standing in cage or on a platform that is swung around.
    The blue N3 and W combined provides the centripetal force.
     
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