1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Sum of forces

  1. Apr 6, 2013 #1
    I'm drawing a cone (black color) inside balls. Balls are pressured from external force, but the pressure is fixed. Balls repuls themselves and cone repuls balls too. The cut view of cone (bot) showing part of balls all around cone, arrows are forces all around balls. For me F1>F2, because like balls are around cone they repuls themselves more than the left side. How can I draw correctly the forces for have 0 ?
     

    Attached Files:

    • dft.png
      dft.png
      File size:
      17.7 KB
      Views:
      65
  2. jcsd
  3. Apr 6, 2013 #2

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    If there was a net force the object would propel itself along, moving through the sea of balls without needing fuel. It's not going to happen.

    The force between a ball and the cone acts perpendicular to the surface of the cone. So although the surface of the cone is larger than the flat end the horizontal component is the same.
     

    Attached Files:

    Last edited: Apr 6, 2013
  4. Apr 6, 2013 #3
    Sure, it's not moving alone in water :approve: I'm ok with the difference of surface too. I show balls like a circular dam that prevent force to apply on cone's surface, like a protective shield (drawing). When cone is moving, balls move closer from each other for "build" a "circle" (following the surface) so the external force can apply all the force ?

    In the drawing, I don't draw all balls, I sould but maybe there is a simple explanation.
     

    Attached Files:

    Last edited: Apr 6, 2013
  5. Apr 8, 2013 #4

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    Sorry. I don't understand what you are asking.
     
  6. Apr 8, 2013 #5
    The last view is a cutting view (a slice of cone). The cone in the center must receive force from balls. Imagine balls which repuls themselves and cone repuls balls. For receive the pressure balls will be like the last view, but like the shape of balls is a circle all the pressure can be apply ? If I apply a pressure on a sphere, the object inside can't receive the pressure (without detroy sphere).
     
  7. Apr 8, 2013 #6

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    So is this the situation? The balls are packed together tightly so they cannot apply pressure on the cone in the middle?

    I think that's unlikely to occur. The balls will rearrange themselves in 3D. I have no idea how to model that.

    It's possible that fewer balls might touch the cone side compared to the flat side - but there is no reason why the force applied by each ball has to be the same.
     

    Attached Files:

  8. Apr 10, 2013 #7
    Yes, this is the situation, but balls are not necessary in contact with cone and with others balls if repulsive force is big compared to the "pressure". I'm trying to simulate with Comsol and say how are balls in 3D.
     
    Last edited: Apr 10, 2013
  9. Apr 14, 2013 #8
    While awaiting the good model for Comsol, I'm thinking with a static problem that it's easier to find my error. Put a cone in a container with a lot of compressible balls. Gravity is like drawing. Choose diameter of balls for have an integer nomber of balls in each layer, like that it's more easy for have sum of forces, fig2. For me the force F2 that come from others balls in circular shape (fig3) change the sum of force for each ball (in contact with cone) and give a up force. Each layer of balls can be separate with a sheet, like that the study is only one layer. The problem exist only with the ball in contact with cone, for others balls I agree sum of forces is 0 (apart weight). Maybe if container is a cone too, it's easier for have sum of forces ?

    where I'm wrong ?
     

    Attached Files:

    Last edited: Apr 14, 2013
  10. Apr 16, 2013 #9
    I understood my error, due to the circular shape of balls: balls want to move up and the cone want to move down. Sum of force is zero. But, in this case if density of cone is the same density than balls, this system don't respect Archimède's principle ? The shape change the Archimède's principle ?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook