Here's a Question! - Masses on a String (Fast 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 quickly, 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 quickly on the handle, where does the string break?
Steve: Does it break at the top, between the dowel and first roll of tape?
Joanna: Does it break in the middle, between the two rolls of tape?
Steve: Does it break at the bottom, between the second roll of tape and the handle?
Joanna: Or, is Steve too weak to break the string?
Pause the video now if you'd like to think it over before we show you what happens.
Steve: When I pull on the handle, I apply a force to the entire stack. The stack accelerates, but it's a lot easier to accelerate this little bit of string than it is to accelerate these two massive rolls of tape. The tension on the lower string reaches its breaking point before the rest of the stack has a chance to react. Basically, the lower string out-accelerates the rest of the stack.
Joanna: Imagine you tie a string to an elephant on rollerskates. Pulling on the string breaks the string before the elephant even begins to roll. It's the same thing with this. Pulling too quickly on the string breaks the string.
Thanks for watching. I hope you'll join us again soon for another question!
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