How to prevent game object from rolling on a curved plane?

[ZG_UCSC]

I have created a twisted plane by adjusting the vertices of the mesh in a plane object. I am trying to make other game object going over it smoothly, i.e. making full surface contact with the plane and with local y axis in the same direction as the plane’s normal. Here is what I have for now:

As you might see from the image above, the game object, which is a cube, is rolling over the curved plane surface instead of sliding over it. How to prevent such undesired rolling? Here is the script I have for the cube now:

using UnityEngine;
using System.Collections;

public class CubeControl2 : MonoBehaviour {

    private Rigidbody rb;
	void Start () {
        rb = GetComponent<Rigidbody>();
	}

	void Update () {
        rb.velocity = new Vector3(0, 0, -1);
        Vector3 gravity = new Vector3(-transform.localPosition.x, -transform.localPosition.y, 0);
        rb.AddForce(gravity);
	}
}

And here is the script I used to twist the plane:

void Awake()
{
    Mesh PlaneMesh = GetComponent<MeshFilter>().mesh;
    Vector3[] vertices = PlaneMesh.vertices;
    for (int i = 0; i < 11; i++)
    {
        for (int j = 0; j < 11; j++)
        {
            if (j < 5)
            {
                vertices[i * 11 + j].y = vertices[i * 11 + j].x * Mathf.Sin(Mathf.Deg2Rad * 18 * i);
                vertices[i * 11 + j].x = vertices[i * 11 + j].x * Mathf.Cos(Mathf.Deg2Rad * 18 * i);
            }
            else if (j > 5)
            {
                vertices[i * 11 + j].y = vertices[i * 11 + j].x * Mathf.Sin(Mathf.Deg2Rad * 18 * i);
                vertices[i * 11 + j].x = vertices[i * 11 + j].x * Mathf.Cos(Mathf.Deg2Rad * 18 * i);
            }
        }
    }
    PlaneMesh.vertices = vertices;
    GetComponent<MeshCollider>().sharedMesh = PlaneMesh;
}

What should I change?

[Naphier]

Ultimately, rigidbody might not be the best option. I had to do something similar for a luge game. What I ended up doing is adding an anchor to my rigidbody to give it more weight below the surface. It might work just the same if you up the mass and gravity, also try changing the angular drag a bit higher.

[b1gry4n]

The “slipping” is caused by gravity I am assuming. Gravity is a constant force (default is 0, -9.81, 0) so it will always be forcing objects in that direction. I am not sure of the exact way gravity is applied to rigidbodies, but when I use rigidbodies, i never use the default gravity. I always disable rigidbody gravity and create my own gravity, especially when I want complete control over an object in weird situations.

    void FixedUpdate()
    {
        rigidBody.AddForce(Physics.gravity);
    }

What you could do is use this same method, but change the direction of the force so that it is always pointing at the “point of contact”. You could get the point of contact with a ray cast or a direct reference to the game object. Ill leave figuring that part out to you.

    void FixedUpdate()
    {
        Vector3 dir = (transform.position - pointOfContact).normalized;
        rigidBody.AddForce(dir * Physics.gravity.y);
    }

If i have misunderstood your question and you want to prevent the square from simply tumbling over, dont let the rigidbody control rotation when the object is “grounded”. Lock the rotation and rotate the rigidbody yourself. If the object isnt grounded, give control back to the rigidbody