How force works on two strings connected by a mass

[Ezio3.1415]

So if I slowly pull the A string first B string will tear apart... Cause the A string will expand and it will pull the mass and B string... So B string will reach its breaking weight before A... Cause B has an additional weight mg working on it..

But if I suddenly pull the A string,it will tear apart but the B string will remain intact... I am trying to understand why this happens... I mean what is the exact mechanism here?

 

[dauto]

Applying Newton's second law to the block we have

F_net = ma
F_A - F_B + mg = ma
F_A - F_B = m(a-g).

If you pull it slowly, acceleration a is negligible and we get

F_A - F_B = -mg < 0
F_A < F_B

F_B is larger.

But if you pull very fast than a is very large and we get

F_A - F_B = m(a-g) >0

F_A > F_B

F_A is larger.

 

[Ezio3.1415]

Thank you... :)