Friction defined in contacts vs geoms #2862

aaprasad · 2025-09-19T18:37:58Z

aaprasad
Sep 19, 2025

Intro

Hi!

I am a graduate student at MIT, I use MuJoCo for my research on neuromechanical simulations.

My setup

mujoco 3.6.0

My question

I'm trying to emulate a worm sliding on the floor and I'm messing with the friction parameters. I'm a bit confused as to the difference between defining the friction in a contact pair vs in the geom directly. I saw in the docs it states

The reason for having 5 coefficients per contact and only 3 per geom is as follows. For a contact pair, we want to allow the most flexible model our solver can handle. As mentioned earlier, anisotropic friction can be exploited to model effects such as skating. This however requires knowing how the two axes of the contact tangent plane are oriented. For a predefined contact pair we know the two geom types in advance, and the corresponding collision function always generates contact frames oriented in the same way – which we do not describe here but it can be seen in the visualizer. For individual geoms however, we do not know which other geoms they might collide with and what their geom types might be, so there is no way to know how the contact tangent plane will be oriented when specifying an individual geom. This is why MuJoCo does not allow anisotropic friction in the individual geom specifications, but only in the explicit contact pair specifications.

So basically my understanding is that to define anisotropic friction you have to do so in a pair by setting the first two elements of the friction to be different. However let's say I have a floor geom and a capsule geom defined like:

<body name="table0" pos="1.32441906 0.75018422 0.2">
			<geom size="0.625 0.45 0.2" type="box" condim="3" name="table0" material="table_mat" mass="2000" friction="0.1 0.005 0.0001"></geom>
		</body>

		<body name="object0" pos="0.025 0.025 0.02">
			<joint name="object0:joint" type="free" damping="0.01"></joint>
			<geom size="0.025 0.02" type="cylinder" condim="3" name="object0" material="puck_mat" friction="0.1 0.005 0.0001" mass="2"></geom>
			<site name="object0" pos="0 0 0" size="0.02 0.02 0.02" rgba="1 0 0 1" type="sphere"></site>
		</body>

would this be equivalent to doing

<contact>
		<pair name="table_object" geom1="table0" geom2="object0" condim="3" friction="0.1 0.005 0.0001"/>
</contact>

Minimal model and/or code that explain my question

No response

Confirmations

I searched the latest documentation thoroughly before posting.
I searched previous Issues and Discussions, I am certain this has not been raised before.

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

Friction defined in contacts vs geoms #2862

Uh oh!

{{title}}

Uh oh!

Replies: 0 comments

Select a reply

Uh oh!

Friction defined in contacts vs geoms #2862

Uh oh!

aaprasad Sep 19, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 0 comments

aaprasad
Sep 19, 2025