# yade-users team mailing list archive

## [Question #694390]: sphere-facet contact

```New question #694390 on Yade:

Hi all,

As we all know, the timestep is calculated by p-wave analysis for the sphere-sphere/sphere-facet contact[1].

The last time, I posted a qeustion[2], I said that the reason for the particle blows up is related to the timestep. when the number of the penetrator segments is 10, the simulation runs well, while when the number of the penetrator segments is 50, the particles blow up. (the other parameters are the same, except for the number of the segments). This means the timestep for these two simulations should be the same. why the particles blow up?
######################################################
I still didn't fully understand the situation of the sphere-facet contact.
######################################################
(1) If a sphere contact with the facet, there is one contact point if the central point of the sphere is located within the triangular area of the facet.
(2) There are two contact points if the sphere contact at the edges of two neighboring facets (In convex edge). At the same time, there are two contact forces, one is from the sphere-facet1, the other is from sphere-facet2.
(3) Sometimes we said that the contact normal is perpendicular to the facet. the precondition for this is that the contact is within the triangular area of the facet. If the contact point is between the sphere and the edge (which is made by two facets), the contact normal will not be perpendicular to the facet.
(4) As for the vertices, what are the differences if the cone is made of 10 segments and 50 segments?  (the higher the segments, the higher the resultant force on the vertice???)
(5) Jan mentioned that it is like the cone tip has 50x higher stiffness. But actually, the stiffness is still the same, right??? If this is correct. how do we determine the timestep for the sphere-facet contact?

overall, there might be two or more contacts between the sphere-edge/sphere-vertice contact.
###################################################################
I also read some articles about pfacet[3]. Here, it mentioned that there is only one contact between the sphere and the edge (convex edge).
############################################
Do we have any detailed information about the sphere-facet contact? where I can understand it thoroughly.

References: