Setting gravity to newly created entity

[tomhallam]

Hi there,

I hope this isn't a bad question, but I couldn't really understand your application of forces in the gravity tutorial.

With the following code:

                    var obj = _this.world.addEntity({
                        imgSrc: 'enemy.png',
                        radiusPixels: 20,
                        xPixels: Math.floor(Math.random() * canvas.width),
                        yPixels: 0,
                        //angleRadians: 45,
                        angularVelRadians: 10
                    });

How can I set the gravity of the newly created object to something like 0.5?
obj.body.ApplyForce(0,0) // for example, made everything mess up

I don't really understand the concept at the moment, a quick explanation would get me back on track.

Thanks for the great library :)

[jcole]

Hi, the gravity is set for the world in general. In the Gravity demo, gravity is set to 0 in both the X and Y-directions.

What makes the objects actually attract each other is calculating the force between them based on the formula:

force = some_constant * mass of object1 * mass of object2 / (distance between them)^2

  # some low-level Box2d action here
  applyGravities = (obj1, obj2) ->
    pos1 = obj1.body.GetWorldCenter()
    pos2 = obj2.body.GetWorldCenter()
    diffVec = pos2.Copy()
    diffVec.Subtract(pos1)
    distSq = diffVec.LengthSquared()
    forceMagnitude = gravitationalConstant * obj1.body.GetMass() * obj2.body.GetMass() / distSq
    diffVec.Normalize()    
    diffVec.Multiply(forceMagnitude)
    obj1.body.ApplyForce(diffVec, pos1)