Creating a movie clip :
    Create new flash document.


    Draw a circle using the oval tool.


   

    Select the circle using double click. Press F8 to convert to symbol.


    Part 1 :
   
       Click on the circle and Press F9. Actionscript Panel will open. Insert the following actionscript.

onClipEvent(enterFrame){
    diffx = _root._xmouse - this._x ;
    diffy = _root._ymouse - this._y ;
    _x += diffx ;
    _y += diffy ;
}



    Explanation about the code

_xmouse :   returns the x coordinate of the mouse.
_ymouse :  
returns the y coordinate of the mouse.
_x            :  
An integer that sets / stores the x coordinate of an object.
_y            :  
An integer that sets / stores the y coordinate of an object.

    variable diffx & diffy is used to store the distance between mouse's x & cordinate and object's x & y cordinate respectively.
     _x += diffx ;
    Assigns _x , the value of _x + diffx .
    _y += diffy ;
    Assigns _y , the value of _y + diffy .



    Part 2 :

replace

_x += diffx ;


_y += diffy ;


with

_x += diffx / 5 ;

_y += diffy / 5 ;


Explanation of code :

_x += diffx / 5 ;
    Assign _x , the value of _x + diffx / 5 ;( dividing the diffrence with a number decreases the speed of movment. Higher the number lower the speed. )
_y += diffy / 5 ;
    Assign _y , the value of _y + diffy / 5 ;( dividing the diffrence with a number decreases the speed of movment. Higher the number lower the speed. )