Drag physic object in corona sdk

I'm try to drag a dynamic body with gravity = 0,0 in my scene, I have a square with body type dynamic, and an image with body type static, but when drag the square over the image this have a little force but can exceed the image and pass to the other side like the images:

This is my code to drag the square:

  local function dragBody( event )
                  local body = event.target
        local phase = event.phase
        local stage = display.getCurrentStage()

        if "began" == phase then
                stage:setFocus( body, event.id )
                body.isFocus = true
                body.tempJoint = physics.newJoint( "touch", body, event.x, event.y )

        elseif body.isFocus then
                if "moved" == phase then
                        body.tempJoint:setTarget( event.x, event.y )

                elseif "ended" == phase or "cancelled" == phase then
                        stage:setFocus( body, nil )
                        body.isFocus = false
                        body.tempJoint:removeSelf()

                end
        end
        return true
end

And this is the code to create the objects:

function scene:createScene( event )
    local group = self.view
    my_square = display.newImage("square.png")
    my_square.x = 60
    my_square.y = 60
    physics.addBody(my_square, "dynamic" )
    group:insert(my_square)

    floor = display.newImage("piso.png")
    floor.x = 160
    floor.y = 240
    physics.addBody(floor, "static" )
    group:insert(floor)   
end

thanks for help.