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Rigidbody Attraction Orbit

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#1 crueltiesladle   Members   -  Reputation: 113


Posted 02 June 2014 - 05:31 PM

I'm working on a game in unity and I've stumbled upon a physics problem that I don't really understand how to fix.




In this image, there are objects A and B.  Let's just say they both have a mass of one.


As object A approaches object B, and the max distance has been breached, I want object B to attract quickly to object A's Min distance and then simply stay still (unless opposing outside forces choose to intervene).  


This way when the object gets close enough to other objects it can affect, it will pick them up and bring them along on the journey.


Would this problem be solved if I used a simple rigid spring thats resting position is the min distance?


Or is there another better way for solving this problem like giving a force to both objects so that their equlilibrium would be met when the min distance is achieved.  


So far what I've tried causes weird orbits and jitter.

Edited by crueltiesladle, 02 June 2014 - 05:31 PM.


#2 CryoGenesis   Members   -  Reputation: 484


Posted 02 June 2014 - 06:13 PM

Off the top of my head, the solution I would try would be to have a constant gravitational-type force between the two objects then multiply the accelaration amount by a variable multiplier, which can be changed based on the distance from the object.


So an algorithm like this could work:

double multiplier = 1;

if(distance < minimum){

    multiplier = -1;
    //Repulsion force

}else if (distance > minimum && distance < speedUpDistance){

    multiplier = 1;
    //Strong attraction

} else if (distance > speedUpDistance){

    multiplier = 0.1;
    //Weak attraction


velocity += (multiplier/distance) * genericGravityConstant
position += velocity

It would basically get pulled towards the rigid body, then repulse, then pull again. If you take some of the momentum away each update of the simulation then it may reach a fixed resting point until as well.


Not sure if this would work, it's pretty late where I am.

#3 crueltiesladle   Members   -  Reputation: 113


Posted 02 June 2014 - 08:25 PM

Yeah thats basically the implementation I have currently.  So far this doesn't quite do what I want it to.  How i'd like it to act would ideally be a rigid connection between to two objects upon getting to the min distance, and then having a minimum force that it needs to achieve in order to break off from that connection.  How do you make a stiff connection between two entities using only forces?

#4 CryoGenesis   Members   -  Reputation: 484


Posted 03 June 2014 - 02:45 AM

Using the same way I described could be used to create a really stiff connection but it would need to be changed. It wouldn't be a solid connection, per say, but to the player it would look pretty solid, in my opinion. 


First of all, the multiplier would have to changed using an equation based on distance, instead of 3 set values so that the bodies are held at a certain distance, which would be the minimum distance. The nice thing about this method is that the minimum force required to separate the objects would be just be anything bigger than the force holding them together. 


So something like this:

if(distance < minDistance){

    force = -connectionStrength / (minDistance-distance);

else if(distance > minDistance && distance < maxDistance){

    force = connectionStrength / (minDistance-distance);

else if (distance > maxDistance) {

    force = 1/distance;


This may not work properly because you're dealing with acceleration. I think the rigid body might bounce around. I'll have to test this myself and get back to you. 

#5 hagerprof   Members   -  Reputation: 121


Posted 04 June 2014 - 08:18 AM

You might also want to create a small buffer zone to reduce jitter:


if (buffer > abs(minDistance-distance) {

      velocity=someConstant   // or  0


#6 crueltiesladle   Members   -  Reputation: 113


Posted 05 June 2014 - 01:50 PM

Awww yeah I did it.  Implemented both solutions you guys offered and it looks great now.

Thanks a lot.

#7 hagerprof   Members   -  Reputation: 121


Posted 06 June 2014 - 11:00 AM

No problem.  Good luck on your project!