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.

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.

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.

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.

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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