# DEM contact models¶

In DEM, particles are represented as rigid geometry and any deformation of particles due to collisions with other ones are accounted for by contact models. Modeling contact consists (roughly speaking) of 3 tasks:

1. Decide whether particles geometrically interact, and if so, then:

1. compute/update local contact coordinates, defined by contact point, contact normal (local $$x$$-axis) and orientation of the perpendicular plane (local $$y$$- and $$z$$-axes);

2. compute relative velocities of particles in the local system (woo.dem.L6Geom.vel and woo.dem.L6Geom.angVel)

2. Combine material parameters and compute non-geometrical properties of the contact, such as stiffness or friction parameters. This is usually done only once, when the contact is created (woo.dem.ContactLoop.updatePhys).

3. Compute effects of the contact on particles – force, torque) etc.

The first task is covered in the Geometry chapter while the last is covered in the following ones. The second task, computing contact properties from material parameters, is an implementation detail at this point not dealt with here.

Tip

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