Processing.js physics simulation
function Constructor(){
  this._isMyPrivates = function(){}
}


The usual problem with this "function inside this constructor to emulate private" approach is that every instance of the constructor will have copies of the functions. This will eat up your memory fast, especially that your object sounds like a base class for physics objects.

Try using prototypal inheritance instead.The methods are defined once, and shared rather than defined for each instance.

// Constructor
var PhysicsEntity = function(mass, entityWidth, entityHeight, accelerationVector, position, lifeTime){
    this.mass = mass;
    this.entityWidth = entityWidth;
    this.entityHeight = entityHeight;
    this.accelerationVector = accelerationVector;
    this.position = position;
    this.velocity = new PVector(0, 0);
};

// Prototype
PhysicsEntity.prototype._calculateForces = function(){}
PhysicsEntity.prototype._applyForces = function(){}
PhysicsEntity.prototype._renderEntity = function(){}
PhysicsEntity.prototype.updateEntity = function(){}

// This means that
// instance (where props live) -&gt; prototype parent (where methods live)
//                                      A
// another instance (where props live) -' // literally pointing to the same object


And don't worry about "private". One can easily tamper the JS anyways, one way or another. You can prefix privates with underscores, as it's the usual convention.