A stone hangs by a fine thread from the ceiling and a section of the same thread dangles from the bottom of the stone. If a person gives a sharp pull on the dangling thread, the bottom string will break, but if he slowly pulls the bottom string the top string will break. After seeing this demonstration on video, I am still confused on the principles that are involved here. Exactly why do these things occur (the breaks in the string at various times) in reference to the difference in tensions and a change in acceleration? I attempted to do a force diagram to help with this question: ---| ---| Tension 1 --rock ---| ---| Tension 2 (applied force) Another arrow (force) also points downward for mg. My teacher mentioned the formula T2-T1 + mg = ma, but I do not understand why mg is not -mg (T2 - T1 - mg = ma). Pertaining to the reasons why, I did some readings on the subject and they mentioned that the top thread experiences the tension due to the weight of the rock in addition to the force exerted by pulling the bottom thread. The bottom thread breaks due to the fact that the rock has inertia resisting a sudden change of its velocity. I still don't truly get the difference in tensions and change in acceleration that lead to the threads breaking. I would really like to understand this experiment. Please explain the things that I mentioned to me in a simple but thorough manner. (I'm sorry if I sound really ignorant.) Thank you.