In completing Walter Lewin's 6th lecture on Newton's Laws, he presents an experiment (go to 42:44) which leaves me baffled.
(I recommend watching the video; see link above.)
- There is a $2$ kg block with 2 identical strings attached to it: one at the top, the other at the bottom.
- The top string is attached to a "ceiling", and the bottom to a "floor".
- Professor Lewin "stretches" the system (by pulling on the bottom string) with the block not accelerating.
- One string snaps.
- Initially, the top string has a tension of approximately $20$ N, to counter the force of gravity. The bottom string has no tension at all.
- Then, when Lewin pulls the bottom string, it gains some tension $n$ N. To counter act the force exerted by the bottom string, the top string exerts now $20 + n$ N.
- I assume that the string with more force will give out sooner, leading me to conclude that the top string will break.
(This was conducted by Lewin, not me; see link above.)
- Trial 1: Bottom string breaks.
- Trial 2: Top string breaks.
- Trial 3: Bottom string breaks.
The results don't seem consistent. If I was right, I'd expect all 3 experiments to be right; conversely, if I was wrong, I'd expect all 3 experiments wrong, with one exception: the results are more-less random and one result isn't preferred over the other.
- Why was my prediction incorrect?
- Was there a flaw in my logic?
- Why were the results inconsistent?