Stopping Rigidbodies with Opposing Forces

Questions & Answers
1

Hi

I’m currently toying with Unity’s physics system and have been trying to use an opposing force to stop an object. I’m not that great at physics so let me know if I am doing it wrong. Here’s my code

function FixedUpdate()
{
    if(Input.GetButton("Stop"))
    {
        doStop();
    }
}

function doStop()
{
    var f:Vector3  = -(rigidbody.mass * rigidbody.velocity);
    rigidbody.AddForce(f, ForceMode.Impulse);
}

It seems to work to a limit so I think I am doing something right but the object moves in its original direction ever so slightly. Any thoughts to why this is happening?

3

You use ForceMode.Impulse so the effect will be immediately. If you relly want to stop it just zero the velocity :wink:

rigidbody.velocity = Vector3.zero;
4

Ok so I’ve solved the problem. I needed to treat gravity separately and apply a force opposite to gravity. Here’s the code. Hope it helps others.

function FixedUpdate()
{
	if(Input.GetButton("Stop"))
	{
		doStop();
	}
}

function doStop()
{
	var f:Vector3  = -(rigidbody.mass * rigidbody.velocity);
	rigidbody.AddForce(f, ForceMode.Impulse);
	rigidbody.AddForce(-(Physics.gravity), ForceMode.Acceleration);
}
5

Thought I would rephrase my solution because some might get confused why I didn’t just set rigidbody.useGravity = false;. I avoided this so I could use damping on the effect like so

private var strength:float = 0;
private var smoothTime:float = 2.0;

    function FixedUpdate()
    {
    	if(Input.GetButton("Stop"))
    	{
    		doStop();
    	}
    	if(Input.GetButtonUp("Stop"))
    	{
    		strength = 0;
    	}
    }
    
    function doStop()
    {
    	Mathf.SmoothDamp(0.0f, 1.0f, strength, smoothTime);
    	var f:Vector3  = -(rigidbody.mass * rigidbody.velocity) * strength;
    	rigidbody.AddForce(f, ForceMode.Impulse);
    	rigidbody.AddForce(-(Physics.gravity) * strength, ForceMode.Acceleration);
    }
2

rigidbody.velocity is accidentally the right value (since force is mass x acceleration, and you want to accelerate to a zero velocity). I think imperfection in floating point math will result in some residual movement. I think you’ll need to add a script that’ll dampen / eliminate the resulting movement. So anything in your system traveling slower than some “minimum” speed will stop.