Quantitative estimation of the famous trick with two identical strings and a block

RafaelPetros

Dear friends,

here is the problem which I would like to solve. pleas watch this YouTube video from the 43rd minute http://www.youtube.com/watch?v=M_8w-...feature=relmfu

It is clear why it is happening so, however I would like to have some quantitative estimation for this problem. For instance for given strings with critical tension "T(critical)" and for the block with mass "M" I would like to know in which case upper sting will be broken and in which case lower string will be broken for
constant velocity pulling (v=const)
for constant force pulling (F=const, ∂v/∂t=const)
linear force pulling (∂F/∂t=const, ∂(∂v/∂t)/∂t=const)

Thanks in advance.