Brief function question


function animateDown(target_Y:Number, target_mc:String, speed:Number) {
    //i want to reference the target_mcs _y
}


I have this function definition in a keyrame on the _root timeline. It chucks an error if i try to set the

target_mc._y

how can i set up my function so it can be passed any target_mc from anywhere and change its _y? wahts the syntax?