What you Need
What to do
What may happen
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.
Why does it happen?
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.
This means that the upper piece of cotton should always break first.
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.
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.
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.
Similarly it is possible to tear loo-paper one handed using a sharp pull - either it tears or it unravels a lot! ;D Brax, Fri, 17th Feb 2012