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This question already has an answer here:

Imagine a fragile cord attached to a ceiling with its other side attached to a sphere. On the lower part another cord is atrached to the sphere. Now visualize you pull STRAIGHT downward on this cord. Where (i.e. at the upper or lower rope) will the cord break if you pull with much force quickly and where will it break if you pull for a longer duration with less magnitude? My assumption for the first, is that it would break at the lower rope, as it seems to me that inertia is the answer to this question here.

Curious about your valuable opinions! Thanks in advance.

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marked as duplicate by sammy gerbil, stafusa, Jon Custer, John Rennie newtonian-mechanics Sep 3 '18 at 6:48

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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This is a commonly used demonstration.

With a sharp tug the bottom thread breaks because the hanging mass moves hardly at all thus not stretching the top thread very much and thus the hanging mass is effectively a fixed support.
This results in tension in the bottom thread being larger than that in the top thread.

With a slow tug the hanging mass has time to move significantly and stretch the top thread.
As the tension in the top thread is equal to the weight of the mass and the tension in the bottom thread it is the top thread which breaks.

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