|MadSci Network: Physics|
This is actually a very difficult question to answer. It's also a very important question to people that design buildings, bridges, cars, or any other object where it is important to understand "mechanical failure" (the engineering term for something that breaks). A "part" always breaks at the weakest point, but it is very important to know how the "breaking" force is applied. In your experiment, if you gave the bottom string a sharp, fast, yank, the weight in the middle would protect the upper string because of it's inertia and probably force the lower string to break at its weakest point. If you gave a slow, gentle tug until something broke, you would have the break occur at the weakest point of either string. In your case, where you re-used the string. The act of pulling may have weakened the upper string, causing it to be the "weakest point" in your second test. You also weakend the lower string, so I think in this case, the upper string broke by chance. I would suggest you try the demonstration again and see the difference between fast and slow "yanks". You could also try doing the yanking through a spring scale. By reading the scale, you will measure the amount of force you applied. Have fun with your experiment. Chris Seaman Senior Staff Engineer Alcoa Technical Center
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