Rigidbody unwanted drifting

Questions & Answers
1

Hi everyone!
yet another question on physics stuff…

I’ve got a simple scene with a cube and a plane.
The cube has a box collider and a rigidbody attached while the plane has just a mesh collider attached.

I made a simple script to make the cube slide on the plane, according to user input, that only applies forces to its rigidbody.
I used the following code to make it move on its forward direction:

float vaxis = Input.GetAxis("Vertical");
rigidbody.AddRelativeForce(speed * vaxis * Vector3.forward);

Drag and angular drag are set to zero.

Since there are no physic materials attached to any object in the scene and the physics settings are set to default values, I would expect the cube to slide in its forward direction without changing its orientation but this is not what I get.
Its rotation params slightly change and the result is that the cube is not moving straight forward, but is following a curved trajectory on the plane that I can not control.

If gravity is disabled on the cube or if I freeze rotation, everything works as expected but this is not a solution in my case.
Why is this happening? Am I missing something in the way rigidbody or physics works?

Any help is appreciated!

EDIT -----------

I forgot to mention that I set the cube’s rigidbody center of mass to (0, -1, 0) to make the cube slide and not roll…

3

I’ve run into this problem and yes, setting dynamic friction to 0 stops it, but this is not accurate to the real world.

I also seems the higher the friction the more forceless sliding takes place. Again, opposite to what one would expect!

2

Yes this effect is very much expected (basic Physics). The friction force is making the cube to rotate. So you have two options

1.Freeze Rotation

2.Add a physics material( set friction to 0 and rest options to minimum) and the material to box collider (i.e cube ).

4

This was my problem.

Another solution is to put a rectangular collider on the bottom part of the object so that it doesn’t spin. At least that’s what worked for me. Yes I realize this is 6 years old but hey if I’m reading it someone else will be too. Good luck!

5

I also encountered two similar problems. One is a robot similar to a biped robot. When the left and right input signals are the same, the robot’s forward direction will be deflected; the second is an underwater drone. When the robot rotates in an unstable pitch direction, the robot will appear in the front and rear directions drift.