Which Thread Breaks? Using inertia to your advantage

29 January 2012






2 pieces of Cotton about 40cm long


A bottle

A small 250ml plastic bottle of water or drink




  • Tie both pieces of cotton to the neck of the bottle.
  • Tie the other end of one of the pieces onto something solid so the bottle will hang down.
  • Hold onto the other piece of cotton, and pull slowly. Where does the cotton break?
  • Repeat the experiment, but this time pull very quickly, where does the cotton break now?



You should find that when you pull slowly the cotton breaks above the bottle, but rather surprisingly when you pull quickly it breaks below it.



Pulling slowly

When you pull slowly on the lower piece of cotton, the tension in the upper piece of cotton will always be more than the lower one, because it has to support the weight of the bottle.

 Pulling slowly on the cotton

This means that the upper piece of cotton should always break first.


Pulling Quickly


If you pull quickly, it behaves very differently and breaks the lower piece of cotton.  This is because the cotton is slightly stretchy so to break the upper piece of cotton you have to stretch it.  In order to do this you must move the bottle.

The bottle has mass and so moving it requires a force.  To start with the tension in the lower piece of cotton goes into moving the bottle rather than pulling on the cotton above the bottle.


Quick pull 1Quick Pull 2
When you pull quickly the force goes into accelerating the bottle rather than into the top piece of cotton.This means the force in the lower piece of cotton gets large enough to break it, before the bottle moves very far.


This means that if you increase the force in the lower piece of cotton quickly enough it will break before the bottle has moved very far, meaning the tension in the top piece of cotton doesn't increase much.

Over the next few tenths of a second the upper piece of cotton will pull the bottle back to its original place.  This means the bottle will spread out a short sharp force capable of breaking cotton into a longer, smaller force that will not break the cotton.


Quick Pull 3
The cotton then pulls the bottle back to the original position


Large but short lived forces, known as shock loading, can cause huge problems for engineers as they can break a structure that is otherwise very strong.  Shock loading means the incredibly strong cables used to tow ships can break if they are suddenly pulled tight. 

Shock loading can also be useful; when you hammer in a nail you are applying a very quick huge force to the nail, causing the material you are hammering into to break, and allowing the nail to move forwards.


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