public override sealed void ComputeProperties()
{
float area = Settings.Pi*Radius*Radius;
MassData.Area = area;
MassData.Mass = _density*area;
MassData.Centroid = Position;
// inertia about the local origin
MassData.Inertia = MassData.Mass*(0.5f*Radius*Radius + Vector2.Dot(Position, Position));
}