How to fix/clamp rigidbody dir vector, despite Force dir

I wish to bind a rigidbody to move only along a fixed vector between two points, despite any forces that may be acting on it.

I understand in Maths how to apply the x,y components of a Force to find the resultant on a mass in its direction but I can’t find a way to bind the rigidbody to the fixed vector and prevent it from veering off course, but still respond to the components of physics in its direction.

Imagine a cube with a hole sliding along a pole fixed in space. You can apply a force in any direction you like but the pole fixes movement along one vector and so only the force components in the direction of the vector will produce a movement.

Creating that imaginary pole and fixing the vector is what I’m having the problem with. I require non-K rigidbody so the cube is influenced by a changing gravity.


(Could I achieve this with var Properties?)

So this worked for me. This script is on a non-K rigidbody with collider and ‘Use Gravity’ OFF. I am changing Physics.Gravity with another script but this object only moves in its local Up/Down direction under the influence of the component of Gravity that is acting in that direction Vector. When the object lands on its cylindrical floor or concave ceiling it normally tends to bounce and slide off vector but this script calculates the displacement in Local X axis and adds a force in World Space to correct the displacement.

Blocks are now all aligning wonderfully in my game.
I know this can be condensed but it really helps to see what you can do. (Plus tried a load of stuff that just didn’t work). Also, it didn’t touch the fps even with loads of objects.

#pragma strict
var ImpulseCollisionMax : float = 10.0;
var snapFactor : float = 2.0;

var startVector : Vector3;
var spawnAngle : float = 0.0;
private var rigid : Rigidbody;
	private var gravDot : float = 0.0;
	private var gravX : float = 0.0;
	private var gravY : float = 0.0;
	private var gravZ : float = 0.0;
	private var resultForce : Vector3;
private var startTrans : Transform;
private var startLocalPos : Vector3;
private var localPosition : Vector3;
private var blockRelativeXDiff : float = 0.0;
private var blockDiffVector : Vector3;

function Start ()
	startTrans = transform;
	startLocalPos = transform.InverseTransformDirection(startTrans.position);
	rigid.useConeFriction = true;
	startVector = Vector3(transform.position.x, transform.position.y, 0);
	spawnAngle = Mathf.Atan2(startVector.x, startVector.y) * Mathf.Rad2Deg;

function FixedUpdate ()
	localPosition = transform.InverseTransformDirection(transform.position);
	blockRelativeXDiff = localPosition.x - startLocalPos.x;
	blockDiffVector = startTrans.TransformDirection (blockRelativeXDiff, 0, 0);
	if (blockRelativeXDiff != 0)
	gravDot = Vector3.Dot(startVector.normalized, Physics.gravity.normalized);
		gravX = Physics.gravity.x * gravDot;
		gravY = Physics.gravity.y * gravDot;
		gravZ = 0;//Physics.gravity.z * gravDot;
	resultForce = Vector3(gravX,gravY,gravZ);
	rigid.AddRelativeForce (Vector3.up * resultForce.magnitude * gravDot, ForceMode.Acceleration);	
function OnCollisionEnter(Land : Collision)
    rigid.velocity = Vector3(0,0,0);