Box2d.XNA gravity issue - windows-phone-7

Question

I try to integrate Box2D inside my game for WP7. However, the bodies that I add, do not respond as expected to the gravity. Basically, it seems that whatever property I modify, the object that I add still seems to be very "light" and does not actually respond to gravity changes.
Here is the code:
void Init
{
world = new World(new Vector2(0, 100), false);
world.ContinuousPhysics = true;
// add ground
BodyDef bd = new BodyDef();
Body ground = world.CreateBody(bd);
PolygonShape shape = new PolygonShape();
shape.SetAsEdge(new Vector2(0.0f, bbheight), new Vector2(bbwidth, bbheight));
ground.CreateFixture(shape, 0.0f);
AddObject(new Vector2(450,0));
}
private void AddObject(Vector2 position)
{
float PTM = 32;
Vector2 pos = new Vector2(position.X / PTM, position.Y / PTM);
var circle = new CircleShape();
circle._radius = 1.0f;
var fd = new FixtureDef();
fd.shape = circle;
fd.restitution = 0.5f;
fd.friction = 1.0f;
fd.density = 1000.0f;
BodyDef bd = new BodyDef();
bd.type = BodyType.Dynamic;
bd.fixedRotation = true;
bd.allowSleep = false;
bd.position = pos;
var body = world.CreateBody(bd);
body.CreateFixture(fd);
body.SetUserData(Red);
}
I would be grateful if you could give some help.
Thanks!

Answer 1

Box2D Engine is designed in pixels but in units and it likes small units.
Example if you try scale 1 pixel = 1 unit when you make and object that is 100 pixels wide it is large as a big planet for Box2D. So if the distance between two objects is 300 it will take forever to colide
What you nees to do is to change the scale as Box2D was designed to.
I recommend you to read or watch some Box2D tutorials like this one http://www.kerp.net/box2d/ this tutorial is for Flash Box2D version but main difference is Class names.