The demonstration: A mass of 2kg is hanged to a string that is hanged vertically from a fixture. Another string is attached to the bottom of the mass and then this bottom string is given a downward pull. Note that the strings used are assumed identical. This demonstration can be found here (starting at time 43:00, or the fourty-third mins): http://www.youtube.com/watch?v=M_8w...es/detail/embed06.htm&feature=player_embedded The initial question was, would the top string or the bottom string break first? The answer, according to the demonstration, is apparently that either string could break depending on the way you pull down on the bottom string. The explanation for the top string breaking first isn't too difficult; it is simply due to the fact that no matter how hard you pull down on the bottom string (thus giving it tension), the tension of the top string is always greater than the bottom string by 20N (contributed by the 2kg hanging mass). As for the breaking of the bottom string. I've read through the video comments but none seems to have a correct answer rigorously examined. I've spent a week thinking about the demonstration thoroughly yet I still don't have a confident answer myself. I wish to understand the truth of the matter - Of what cause does the bottom string breaks? - and I need someone to enlighten me here.