Famo.us physics engine: forces and walls not behaving as expected - physics

Question

I am tyring to create a grid of particles, using the famous physics engine. I thought that if 25 particles were placed beteween 4 bounding walls (forming a square) and they all had an equal repulsion force to each other, they would naturally form a grid - ie. they would all be held as far away from each other as possible, given the limits of their world. I expected this to work, even if the particles were just added without an initial position. However, even if I give them an initial position, they don't hold in place unless the force is very small.
Also, I thought that if a wall had a restitution of 0, then a particle would just stop on colliding with it. When I run the code below, I can see particles bouncing off walls. The pen is at:
http://codepen.io/timsig/pen/pJJrOa
What am I failing to grasp? - thanks in advance
define('main', function (require, exports, module) {
// import dependencies
var Engine = require('famous/core/Engine');
var Surface = require('famous/core/Surface')
var Modifier = require('famous/core/Modifier');
var PhysicsEngine = require('famous/physics/PhysicsEngine');
var Particle = require('famous/physics/bodies/Particle');
var Drag = require('famous/physics/forces/Drag');
var RepulsionForce = require('famous/physics/forces/Repulsion');
var Wall = require('famous/physics/constraints/Wall');
var gridItems = [];
var positionsArray = [-140,-70,0,70,140];
var context = Engine.createContext();
var physics = new PhysicsEngine();
var gridR = new RepulsionForce({
strength: 0.015
});
//physics.addBody(planetParticle);
var leftWall = new Wall({normal : [1,0,0], distance : 140, restitution : 0});
var rightWall = new Wall({normal : [-1,0,0], distance : 140, restitution : 0});
var topWall = new Wall({normal : [0,1,0], distance : 140, restitution : 0});
var bottomWall = new Wall({normal : [0,-1,0], distance : 140, restitution : 0});
function gridItemTrans() {
return this.particle.getTransform();
}
function addGridRepulsion(){
var sq1, sq2;
for (var i = 0; i < gridItems.length; i += 1){
sq1 = gridItems[i].particle;
physics.attach([leftWall, rightWall, topWall, bottomWall], sq1);
if ((i + 1) < gridItems.length){
for (var j = i + 1; j < gridItems.length; j += 1){
sq2 = gridItems[j].particle;
physics.attach(gridR, sq1, sq2);
}
}
}
}
function addBodies(){
gridItems.forEach(function(ele){
physics.addBody(ele.particle);
});
}
for (var rows = 0; rows < 5; rows += 1){
for (var cols = 0; cols < 5; cols += 1){
var gridItem = new Surface({
properties: {
backgroundColor: '#23AD23'
}
});
gridItem.particle = new Particle({
position: [positionsArray[rows], positionsArray[cols], 0]
});
//physics.addBody(gridItem.particle);
//physics.attach(centralG, gridItemParticle[rows][cols], planetParticle);
gridItem.modifier = new Modifier({
size: [50,50],
align: [0.5, 0.5],
origin: [0.5, 0.5],
transform: gridItemTrans.bind(gridItem)
});
context.add(gridItem.modifier).add(gridItem);
gridItems.push(gridItem);
}
}
addBodies();
addGridRepulsion();
});

Answer 1

To clear up your understanding, we will fist look at the repulsion being applied.
By Default, the Repulsion decay function in Famo.us is a gravity function (an inverse squared distance decay function). Gravity has a decay based on mass and distance.
var gridR = new RepulsionForce({
strength: 1,
decayFunction : RepulsionForce.DECAY_FUNCTIONS.GRAVITY
});
You can apply a linear function to your repulsion decay and you will create the affect you are looking for.
var gridR = new RepulsionForce({
strength: 1,
decayFunction : RepulsionForce.DECAY_FUNCTIONS.LINEAR
});
If gravity were to decay linearly rather than quadratically, you would need infinite kinetic energy to escape a gravitational field. It would be like living in 2D space.
To answer the second part of your question: The walls have no restitution, but the particles still respond to the force of the other particles.