Stopping Rigidbodies with Opposing Forces

Question

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?

Answer 1

Best Answer

Answer by Blankzz · Jul 06, 2011 at 10:46 PM

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);
     }

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Answer 2

Answer by Bunny83 · Jul 06, 2011 at 07:25 PM

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

 rigidbody.velocity = Vector3.zero;

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Blankzz · Jul 06, 2011 at 07:35 PM 0

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I hadn't thought about doing that thanks. The trouble is if I have another force act on the object but the stop function gets called last the object will stop. I'm not sure how I can tell the order of function calls in unity so I suppose that's another question?

Blankzz · Jul 06, 2011 at 07:37 PM 0

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Just in case you were wondering. In the end the force will ease in so it would gradually come to a stop.

Answer 3

Answer by Blankzz · Jul 06, 2011 at 09:40 PM

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);
 }

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flaviusxvii · Jul 06, 2011 at 10:33 PM 0

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rigidbody.useGravity = false;

shaibam · Jul 06, 2015 at 01:15 PM 0

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How can I apply the power with nicer ease?

Answer 4

Answer by flaviusxvii · Jul 06, 2011 at 07:18 PM

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.

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Blankzz · Jul 06, 2011 at 07:28 PM 0

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Yeah I thought it might be a floating point issue by had to double I was doing it right before proceeding. I think for what I am heading towards stopping the object when the speed reaches a small amount might defeat the purpose of using physics but maybe not. Thanks

Stopping Rigidbodies with Opposing Forces

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