Here's a Question! - Masses on a String (Slow Pull)
A mass is suspended from a wooden dowel by a string. A second mass is suspended from the first with another length of string. Finally, a handle is suspended from the second mass with yet another piece of string. When the handle is pulled down slowly, which string breaks?
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Joanna and Steve: Just science!
Joanna: Hi! I'm Joanna!
Steve: And I'm Steve!
Joanna: Here's a question for you...
Steve: A roll of tape is suspended from a wooden dowel using a piece of string.
Joanna: A second roll of tape is suspended from the first roll of tape using the same type of string.
Steve: Finally, a handle is suspended from the second roll of tape, also using the same type of string.
Joanna: If Steve pulls down slowly on the handle, where will the string break?
Steve: Will it break at the top, between the dowel and first roll of tape?
Joanna: Will it break in the middle, between the two rolls of tape?
Steve: Will it break at the bottom, between the second roll of tape and the handle?
Joanna: Or, will the dowel break?
Pause the video now if you'd like to think it over before we show you what happens.
Joanna: Each section of string is under a different amount of tension. The tension in the top string is equal to the weight of the whole stack while the tension in the bottom string is nearly zero. Clearly, the top string is closer to breaking than the bottom string.
Steve: Pulling slowly on the handle gives the stack time to react to the added force. The inertia of the various pieces doesn't matter so much because, again, there's time for everything to react.
Joanna: Although the tension increases in each of the strings when Steve pulls on the handle, it's always greatest in the top string, so it's the first to reach the breaking point.
Steve: And, in fairness, the dowel could break, depending on how strong the string is. But, with this particular set-up, the string breaks first.
Joanna: Thanks for watching. I hope you'll join us again soon for another question!
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