Processing.js physics simulation

I've been fooling around in JavaScript again, and I've come up with this simple physics simulation. I think it has a pretty simple setup, here's "psuedocode" of sorts to explain it.

• Constructor PhysicsEntity
• Private method _calculateForces - Calculates forces to be applied.
• Private method _applyForces - Apply the calculated forces.
• Private method _renderEntity - Render the PhysicsEntity at it's position.
• Method updateEntity - Update the entity.

I think the interface is pretty clear, but if you have any suggestions, they would be greatly appreciated.

/*
A simple physics object for managing
certain things such as forces acting
upon the object, and rendering.
*/
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);

// Calculate the forces to be applied to the velocity
this._calculateForces = function()
{
};

// Apply velocity to the Entity's position
this._applyForces = function()
{
};

// Render the Entity at it's position
this._renderEntity = function()
{
rect(this.position.x, this.position.y, this.entityWidth, this.entityHeight);
};

// Update the entity
this.updateEntity = function()
{
this._calculateForces();
this._applyForces();
this._renderEntity();
};
};

Here's an example of how this can be used.

// Data for creating new PhysicsEntities and storing them
var DEFAULT_MASS = 1;
var DEFAULT_WIDTH = 10;
var DEFAULT_HEIGHT = 10;
var DEFAULT_ACCELERATION = new PVector(0, 0.2);
var entityArray = [];

// Create a new PhysicsEntity at the mouse
var createEntity = function()
{
entityArray.push(
new PhysicsEntity(
DEFAULT_MASS, DEFAULT_WIDTH, DEFAULT_HEIGHT,
DEFAULT_ACCELERATION, new PVector(mouseX, mouseY),
)
);
};

// Update all entities
var updateEntities = function()
{
for(var n = entityArray.length-1; n >= 0; n--)
{
var entity = entityArray[n];
entity.updateEntity();
}
};

// Main draw loop
draw = function() {
background(255, 0, 0);
createEntity();
updateEntities();
};

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) -> 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.