Science, Maths & Technology

Shanghai Noon

Updated Monday 22nd August 2005

Robert Llewellyn and Dr Jonathan Hare take on Hollywood Science, testing the science that filmgoers take for granted. Here they look at the science behind Jackie Chan urinating on fabric in the film Shanghai Noon

Cotton wool Copyrighted image Icon Copyright: BBC

In this classic sticky-rice western, Jackie Chan finds himself locked up, stressed out and desperate. The year is 1881, and Chinese princess Lucy Liu has been kidnapped in Nevada. Imperial guardsman Chan)has set off to the wild west to rescue her. After hitting it off (literally) Jackie and gunslinger Roy (Owen Wilson) find themselves behind bars.

 

Ancient Chinese teachings dictate that urinating on fabric will strengthen it, and Jackie is able to enact a brilliant escape using a shirt, a wooden stick and, you guessed it, wee. But is this possible? Our two presenters are determined to find out.

To repeat the jail breaking feat, our hosts, being of a somewhat delicate constitution, don’t want to use their own urine, so they construct some using chemicals and household ingredients.

Breakdown of Urine 95% water Copyrighted image Icon Copyright: Used with permission

As you can see urine is 95% water, so what effect does water have on the strength of cotton?

Cotton is 99% pure cellulose. Cellulose molecules form long strands that repeat, forming the fibre that makes up cotton.

Cellulose Molecule Copyrighted image Icon Copyright: Used with permission

Cotton is actually stronger wet than dry. This occurs because the hydrogen atoms in the water create extra bonds with those of the cellulose.

Cotton cellulose has a high degree of polymerization and a high degree of crystallinity.

Crystallinity means that the fibre molecules are closely packed and parallel to one another.

Chrystalline area of a cellulose fibre Copyrighted image Icon Copyright: Used with permission

The higher the crystallinity and polymerization in polymers, the stronger they tend to be. The cellulose chains within the cotton fibres are held in place by hydrogen bonding. These hydrogen bonds occur between the hydroxyl groups of adjacent molecules and are more prevalent between the parallel, closely packed molecules in the crystalline areas of the fibre. When the cotton is wet, the water present forms additional hydrogen bonds increasing the strength of the cotton.

 

Steel rods Copyrighted image Icon Copyright: BBC What makes cotton different from most material made up of cellulose is that the crystallinity in the fibre increases the strength the wetter it becomes. In other material primarily made up of cellulose, like paper, the strength decreases when it becomes wet.

This can be explained in terms of intermolecular hydrogen bonding between cellulose chains and their degree of crystallinity. Hydrogen bonds occur between the hydroxyl groups of adjacent molecules and are more prevalent between the parallel, closely packed molecules in the crystalline areas of the fibre. When the cotton is wet, the water present forms additional hydrogen bonds within the crystalline areas, increasing the strength of the cotton.
The extra hydrogen bonding occurs not only along the cellulose chains, increasing their longitudinal strength, but between them as well, creating an overall stronger structure.

So Jackie Chan was right, cotton is stronger when wet. But merely having a wet shirt isn’t the only reason that Jackie Chan manages to break out of jail. The secret is also in his method.

Chan ties his shirt around the steel bars, places a wooden stick as a handle/lever and turns the handle to bend the bars-amazing! All this is with good old muscle power, so why does this work? We can’t bend steel with our bare hands (well most of us can’t!)

The wooden handle acts as a lever multiplying Chan’s muscle power.

Diagram of knot on bars Copyrighted image Icon Copyright: Used with permission

For the steel bars to bend, the force exerted by the cloth needs to exceed the bending strength of the steel, the turning point will be when the forces are equal.

Force formula Copyrighted image Icon Copyright: Used with permission

Because a is smaller than b, the muscle power (MP) required applied to the wooden stick (lever) will be a fraction (a/b) of that required to bend the steel bars with your bare hands.

Muscle power (MP) = an average man can lift at least half his own body weight, a fit man can do chin-ups and lift his entire weight.

force diagramatically explained Copyrighted image Icon Copyright: Used with permission

We will assume Chan is fit and weighs about 70kg. To lift himself the force he is exerting to overcome gravity (approx 10 metres/second squared) is approx. 700 Newtons (mass x gravity = force).

We will assume he can apply the same force to the wooden stick. Now we need to consider a divided by b.

The radius of our knot will be approximately 3cm and the length of our wooden stick is 60cm (half b is 30cm), therefore a/b is 0.1. By calculation the force we can apply is 7000N. This is equivalent to the weight of a small car or heavy motorbike, which should be enough to bend our mild steel bar. Because the bars are reasonably long, they are also easy to bend. If the bars were shorter, Jackie may have had more of a problem escaping.

Hollywood Science rating 8 out of 10 Copyrighted image Icon Copyright: Used with permission

So Jackie Chan could indeed have escaped from jail, a bit of science to remember if you ever get into a pickle yourself.

 

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