Monday, 31 August 2015

Experimenting with friction in Unity

Initial setup

Let's create a plane. Next create a cube and place it right on top. Add a box collider and rigid body to the cube.

Basic friction

Let's create a Push script adding a force to the cube at every fixed update:


ForceMode.Force indicate a continuous force. The result of applying a horizontal force is interesting: under 10 units (Newtons, I hope) the box doesn't budge. At 10, it doesn't slide, instead, it topples. This strongly suggests that friction is present.

Next we create a so called Physic Material. Default friction value is 0.6 for static and dynamic (see definitions here). At around 0.3 the box is moving, and doesn't topple.

Moving platforms

Turn off the Push script and create a Move script applied to the plane. Since the plane is not a rigid body no point in applying forces. Instead we generate a linear translation inside Update() or FixedUpdate()

transform.position += velocity * Time.deltaTime;

The plane moves, but the box doesn't. So we have friction, but it's a one way affair. Yet the box and plane are still interacting: Once the plane has been removed from under it, the box falls (though, it doesn't topple).

Add a rigid body to the plane and check kinematic. Restart the simulation. Now the plane still slides under the box. Upon reaching the edge, however, the box topples.

Still no friction?

Uncheck kinematic and gravity. This time we get an error: Unity considers that the mesh collider is not convex so it won't allow physics on it. I wouldn't try making the plane convex so I replaced it by a box collider. Without gravity the plane won't fall on its own. However the box is now pushing it downwards. No good.

We don't want this plane to fall so, freeze Y position along with all rotation axis. Now the plane yet again slides under the box, which finally topples.

Still no friction?

Substitute the original Push script to the Move script. Now the plane is dragging the box away (this works with any Force mode).
Finally, re-apply the physic material and lower the friction. Somewhat surprisingly, it now takes significantly low friction values (~0.01) to the box to topple off the plane.

The uncanny experiment

In rigid body settings, freeze the plane's location and rotation (all axis). Run the simulation and observe: the box is moving, then topples off the plane: the plane now behaves like a rolling carpet.

  • Friction is a property of colliders, not rigid bodies.
  • Friction applies to moving rigid bodies using forces: when moving a solid over static terrain, we needn't add a rigid body to the terrain to simulate friction.
  • Friction doesn't apply to kinematic bodies: therefore, to define a moving platform with correct friction:
    • Must have a collider
    • Must have a rigid body
    • Kinematic is unchecked
    • Move the platform using forces.
    • Take advantage of rigid body constraints.

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