Class with physics entity

function Ball:init(colour, x, y, vx, vy, world)
 
	self.world = world
	local filename = ballzFilename[colour]
 
	-- Create ball bitmap object from ball graphic
        local ball = Bitmap.new(Texture.new(filename, true))
 
        -- Reference center of the ball for positioning
        ball:setAnchorPoint(0.5, 0.5)
        ball:setPosition(x, y)
	ball:setScale(2.0) 
 
         -- Create box2d physical object
	local radius = ball:getWidth() / 2
        local body = self.world:createBody{type = b2.DYNAMIC_BODY,
									   fixedRotation = false	
									  }
    body:setPosition(ball:getX(), ball:getY())
    body:setAngle(ball:getRotation() * math.pi/180)
    local circle = b2.CircleShape.new(0, 0, radius)
    local fixture = body:createFixture{shape = circle, density = 1.0, 
									  friction = 0.0, restitution = 0.2}
 
	body:setLinearVelocity(vx, vy)
 
	local torqueDirection = -1
	if math.random() > 0.5 then
		torqueDirection = 1
	end
 
	body:applyTorque(-100 * torqueDirection)
 
	body.type = "ball"
 
 
	body.origx = x
	body.origy = y
	self.body = body
 
 
	self:addChild(ball)	
 
	self:addEventListener(Event.MOUSE_DOWN, self.onMouseDown, self)
	self:addEventListener(Event.MOUSE_MOVE, self.onMouseMove, self)
	self:addEventListener(Event.MOUSE_UP, self.onMouseUp, self)
end

function GameScreen:onEnterFrame(event)
	--
	-- Physics
    self.world:step(1/60, 8, 3)
 
    -- Iterate through all child sprites
    for i = 1, self:getNumChildren() do
 
		-- Get specific sprite
        local sprite = self:getChildAt(i)
 
		-- Check if sprite HAS a body (ie, physical object reference we added)
        if sprite.body then
 
			-- Update position to match box2d world object's position
            -- Get physical body reference
            local body = sprite.body
 
			if body.type == "ball" then
 
				local bodyx, bodyy = body:getPosition()
 
                                -- HACK!!!!!!!!!!!
				-- Apply physics body coordinates to sprite
				sprite:setPosition(bodyx-body.origx, bodyy-body.origy)
 
				-- Apply rotation to sprite				
				sprite:setRotation(math.rad(body:getAngle()))
			end
        end
    end
 
end