# yade-users team mailing list archive

## moment calculation and rotational speed problem

```Hi,Dear all.

I would like to use cohesive contact model to simulate a rock, the small
rock particle is modeled by cementing them and the force assumed by the
cementing is cohesive force--which can assume normal and shear force.  but I
meet problem in moment calculation and the rotational speed calculation:
just for 2D case ( 'x' is cross(), Normal_directior_unit is from P1 center
to P2 center ):
vector_relative_velocity=(Particle1.v+Particle1.omega x
(Particle1.r*Normal_directior_unit) ) -(Particle2.v+Particle2.omega x
(Particle2.r * -Normal_directior_unit) ) ;
vector_relative_shear_velocity=(vector_relative_velocity *
vector_Shear_Direction) * vector_Shear_Direction;
double_relative_shear_velocity=(vector_relative_velocity *
vector_Shear_Direction)
double_ds=double_relative_shear_velocity*dt;

Fs= - Ks*double_ds;
vector_Fs=Fs* vector_Shear_Direction;

Now moment is :
delta_Moment= (Particle1.r * Normal_directior_unit) x vector_Fs; //(Fn is
not calculated here,and also since Fn pass through the center of the mass,
so it has no effect on the rotation)

Total_moment += delta_Moment;

so
Rotational_velocity += ( Total_moment / Inertia)*dt;

My problem met is , after several iterations,
my rotational velocity reachs 1e4 rad/sec, or even more!!! this is totally
incorrect.
So what is the calculation problem in this cohesive condition? ?  or can I
do the moment calculation and rotational speed calculation like this since
there is cementing cohension between particles and the cohension will strict
the free to rotate of the particle now? In the CohesiveFrictionalContactLaw,
I see the moment calculation.