Friday, May 20, 2011

The Physics of Blue Jeans

They're a casual Friday staple. Your dad wears them and your mom wears them. Even babies wear them. They're the uniform of the American teenager. They're blue jeans.


[Jeans are so popular, designer labels are sometimes counterfeited, as seen here. Whether counterfeit or legit, there are some that say the only way to clean your fashion jeans is to freeze them. What does physics have to say about that? Read on to find out. Photo credit: Ben Donley]

Did you ever stop to consider the physics behind this ubiquitous garment? If not, then lucky you! Today, we've done the work for you:

Saddle-wear

Jeans are no stranger to the saddle, being the popular choice for cowboys for decades. The saddle point, a shape known well to mathematicians, is when an object looks like a saddle, curving up in one direction (along the x-axis, if you're a math person) and curving down in the other direction (the y-axis). This can easily be seen in the shape of a Pringles potato chip.

Blue jeans have a saddle point - right in the unmentionables region. The pants curve up to cover the belly and the rump and then also curve down to form the inseam of the right and left pant legs. Why do I care?

Some argue that the universe is a flat endless plane. Others think it is a closed system. (If you traveled in one direction long enough, you would hit yourself in the back of the head.) And some think the universe could be defined by saddle points. So there you are. In the design of your pants is a (maybe) scale model of the universe.

Busting the Myth of Freezing Jeans to Clean Them

Apparently there is a trend of freezing your jeans to clean them. You put your dirty jeans in an air-tight bag, stick 'em in the freezer, come back after a few days and take a whiff. 'The smell is gone', you think. Except, it's not. It will be back.

According to the Department of Agriculture, odor-causing bacteria and molds aren't destroyed when frozen; they're merely inactive. Once the jeans thaw out, the bacteria will turn active again and the odors will return.

This makes sense. We know that vapor pressure - the tendency of a liquid to evaporate or a solid to turn gaseous - is related to temperature. When the temperature decreases, the vapor pressure also decreases. The solid or liquid is less likely to turn to a gas. So, when the jeans cool off, they produce less odor. But as soon as they return to room temperature, they'll be just as smelly as before.

So instead of freezing your jeans, you can try one of two things: 1) Move to the arctic where it's cold all the time and you'll never be stinky, or 2) Pull an old-fashioned trick and air out your jeans on a clothesline.

Durability

On May 20, 1873, Levi Strauss and Jacob Davis were issued a patent for an improvement in fastening pocket openings, or, the reinforcing rivets found on blue jeans. Adding a rivet around pockets and other high-stress areas on blue jeans, Davis discovered, reinforces the seams and makes the jeans more durable.

"The seams are usually ripped or started by the placing of hands in the pockets and the consequent pressure or strain upon them," Davis said in his patent. To reinforce pockets, he suggested using a rivet:
[I]t consists in the employment of a metal rivet or eyelet at each edge of the pocket-opening, to prevent the ripping of the seam at those points. The rivet or eyelet is so fastened in the seam as to bind the two parts of cloth which the seam unites together, so that it shall prevent the strain or pressure from coming upon the thread with which the seam is sewed.
In January, The New York Times did a fashion story in which they compared three pairs of jeans of similar design each made by different manufacturers. They found that it takes almost three times as much force to rip the rear seam on a pair of Levi's than it does on a more costly pair of jeans. I guess Strauss and Davis knew what they were doing.

UV Protection

Those with fair skin may know that it is possible to get a sunburn even through clothes. A plain, white t-shirt, for example, lets through about one-fifth of the sun's rays, giving it an ultraviolet protection factor (UPF) of 5.

Denim, on the other hand, has a UPF of about 1700, blocking a significant amount of skin-damaging ultraviolet radiation from the sun, which gives you a whole new reason to sport the Canadian Tuxedo. (As if you needed one.)

3 comments:

  1. Thank you once again for providing the inspiration for conversation with my jeans loving teen. We had a good laugh about the idea of freezing the smell away (and I think circumvented a potential yucky idea:)

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  2. Thanks for the advice, a lot of people are probably relieved to hear that they don't have to freeze their pants in order to clean them. Putting them on the clothesline looks like a great alternative.

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  3. Nice
    I enjoy visiting here
    I will visit here more often

    ReplyDelete