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Centripetal Force - My conclusions are correct?

  1. Nov 22, 2013 #1
    1. The problem statement, all variables and given/known data

    I'm supposed to give the three forces acting on the basket on a carousel.


    2. Relevant equations

    Principly not for this task.


    3. The attempt at a solution

    When I look at the forces on the baskets on the carousel in constant rotation, I can point out the three forces:

    1. Gravity --> An arrow pointing straight down
    2. Tension in rope or joint ---> An arrow pointing horizontal/up towards the abutment
    3. Centripetal force --> An arrow pointing straight horizontal towards the centre of the carousel


    Is this correct or have I missed something? Many thanks for answers.
     
  2. jcsd
  3. Nov 22, 2013 #2

    Doc Al

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    Good.

    Good. That force acts at an angle.

    Centripetal force is not a kind of force; it's just the name given for the net force causing something to move in a circle. The other real forces are what provides the centripetal force.

    Not sure why they are asking for three forces. Is the basket empty? If there's a person inside, there will be another force.
     
  4. Nov 22, 2013 #3

    CWatters

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    Air resistance?
     
  5. Nov 22, 2013 #4

    Doc Al

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    Good one! (I'm so used to having that ignored that I forgot about it.)
     
  6. Nov 22, 2013 #5

    Yes, of course I forgot that air resistance is involved. I think it wasn't the point in my task, but I will mention it. Another thing I was thinking of was that - as you said earlier - centripetal force is only a resulting force.
     
  7. Nov 22, 2013 #6

    CWatters

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    As you say it might not be what they were thinking of but I can't think of another force to make up the three.

    Edit... Unless it's inertia during starting and stopping?
     
  8. Nov 23, 2013 #7

    adjacent

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    Shuldn't there be an upward force,equal to weight of the the basket on a carousel because it's in equlibrium(I mean not moving down)
     
  9. Nov 23, 2013 #8

    Doc Al

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    The upward force will be provided by the tension in the cable. That force has already been counted.
     
  10. Nov 23, 2013 #9

    adjacent

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    But that force acts at an angle.Then net force can't be zero,it will create another resultant force,So won't it accelerate?
     
  11. Nov 23, 2013 #10

    Doc Al

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    Of course it accelerates. It's moving in a circle!

    The net force along the vertical axis will be zero.
     
  12. Nov 23, 2013 #11

    adjacent

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    I know.but this is due to a horizontal force.

    Think about this:
    attachment.php?attachmentid=64217&stc=1&d=1385224052.gif
    The force equal to the weight is acting at an angle,so it would create a resultant force and accelerate slightly down.Won't it?
     

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  13. Nov 23, 2013 #12

    CWatters

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    No other force needed. The basket flies out and upwards until the vertical component of the tension in the cable is equal to the weight.
     
  14. Nov 23, 2013 #13

    Doc Al

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    The acceleration is due to horizontal net force.

    No. That tension force which acts at an angle is not equal to the weight--it's magnitude is greater than the weight.

    Assuming it moves in a horizontal circle at constant speed, the net force will be horizontal.
     
  15. Dec 8, 2013 #14
    I have another conclusion that I have never read anywhere, but just thinking of: It must be an equilibrium between gravity and centrifugal force, as well as equilibrium between centrifugal and centripetal force. If we focus on the former, it must be that the gravity takes down the force that otherwise would be pointing straight out from centre. Let's say if we decrease the gravity, I believe that the inertia at the object would be the same, but the force straight down (gravity force) would be less - and the equilibrium point would be changed so that the object would be pointing out more, but still the same rotation. Is this correct?

    Two more conclusions:

    The centripetal force (if we pretend this is a real force) is caused by the chain/rope, as a reaction force.

    The centriFUGAL force is caused by the inertia at the object, which strives to be trown out from circle. This is hindered from doing that by the centripetal force. Still, the force that is causing the object to strive out from center is the inertia at the object, called centrifugal force.

    Please correct me if I'm stating anything wrong.
     
  16. Dec 8, 2013 #15

    haruspex

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    It depends on your frame of reference.

    In an inertial frame (not accelerating or rotating), there is no such thing as centrifugal force. Centripetal force is as discussed earlier in this thread and provides the centripetal acceleration.

    In the frame of the object (the basket here), there is no acceleration, by definition. So you need to invent a 'centrifugal' force to balance the resultant of the other forces (the centripetal force). You can alternatively skip mention of centripetal force and consider this a three-way balance between gravity, cable tension and centrifugal force.
     
  17. Dec 8, 2013 #16

    Ok, I was thinking that the centripetal force was WITHIN the cable tension; but only a part of. I'm imaging that the rope is doing two things; 1. Holds the basket up, straight vertical; gravity force. 2. Holds the basket within the circle path, straight horisontal; centripetal force.
     
  18. Dec 8, 2013 #17

    haruspex

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    You could look at it that way here. More generally, though, the gravitational force might not be parallel to the axis of rotation, so you'd need to think of the centripetal force as the radial component of the resultant of all the forces, not as a component of just one of them.
     
  19. Dec 8, 2013 #18
    Ok, I didn't think of when the axis is tilted, sounds complicated. Thanks for giving your opinion about this.
     
  20. Dec 9, 2013 #19

    CWatters

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    No. If the axis of rotation is vertical then those are normally orthogonal to each other (eg at right angles) so they don't need to be in equilibrium.

    Vertically..

    The vertical component of the tension F must be equal to the force due to gravity.

    Horizontally...

    The horizontal component of the tension F must provide the centripetal force required to make it move in a circle.

    The actual tension force F is the sum of these two components.

    Aside: I'm ignoring any force needed to overcome air resistance - that would make it slightly more complicated but the same basic principles would apply.
     
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