This simulation illustrate relations between friction, external force, normal force ...etc.
You can drag the "hand" to move the spring until it touch the object with mass m.
The external force is in horizontal direction only (as default), it can be changed with checkbox.
[flash=799,439]http://www.phy.ntnu.edu.tw/ntnujava/swf/onemassfriction.swf[/flash]