MadSci Network: Physics

Re: Why did the string break differently?

Date: Thu Sep 25 11:17:05 2003
Posted By: Chris Seaman, Staff, Electrical Engineering, Materials Engineering, Alcoa Technical Center
Area of science: Physics
ID: 1063847241.Ph

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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