It’s a question up there with “Why does a kettle make so much noise when it’s boiling?” and “Why is the sky blue?” — and it’s this: Why does your iPhone 4S always fall on the floor when you lay it safely on the arm of a leather chair? The answer’s in the glass …

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Aluminosilicate glass, to be precise. That’s “normal glass doped with aluminum oxides to make it stronger and more scratch-resistant than normal glass”, according to Tested.com. The British Glass web site describes it thus: “A small, but important type of glass, aluminosilicate, contains 20% aluminium oxide (alumina-Al2O3) often including calcium oxide, magnesium oxide and boric oxide in relatively small amounts, but with only very small amounts of soda or potash. It is able to withstand high temperatures and thermal shock and is typically used in combustion tubes, gauge glasses for high-pressure steam boilers, and in halogen-tungsten lamps capable of operating at temperature as high as 750°C.”

Is it Corning‘s famous Gorilla Glass? Corning provides the glass used in a host of smartphones, but there’s no mention of the iPhone at its web site — though it adds that “Due to customer agreements, we cannot identify all devices that feature Gorilla Glass. Your favorite device may include Gorilla Glass, even if you don’t see it listed.” I rather like to think it is Gorilla Glass in my iPhone, because a) I like gorillas, b) Corning is a global expert in this field, and c) Apple likes to go with the best it can get. (The jury is still out for many reviewers because Apple is so coy about its products and processes, but according to AppleInsider, it seems the iPhone 4 does have Gorilla Glass — at least on the front — while the back is aluminosilicate glass, but not the Gorilla brand. Whether that’s the same for the 4S, I don’t know. This Mashable article discusses the use of Gorilla Glass in the iPhone 4S, too.)

So what’s this got to do with the title of this piece? Well, when I arrived home with my new iPhone 4S the other day, I parked it on the arm of my leather sofa and went to another room. About five minutes later I heard an ominous clatter — and returned to find my iPhone on the floor (a hard laminate floor, I might add). I was sure I’d left it safely cushioned on the top of the wide arm — and yet, I supposed, I couldn’t have done, because there it was on the floor. Fortunately, no damage had been done, so this time I put it on the seat.

Fast-forward to last night. Watching TV while sitting on the sofa, and forgetting what had apparently happened the other day, I put my iPhone next to me on the leather arm, alongside the TV remote and my cigarette lighter (obviously, I can’t photograph the phone in place on the arm, because I used the phone to take the photo — you’ll have to use your imagination). About five minutes later, while engrossed in the programme, I suddenly became aware that the phone had slipped down the arm and landed next to my leg on the sofa seat.

I found this very curious. With the phone at rest on the arm, a fair amount of friction is present between the two, meaning that to move it, a substantial push with a finger is needed to overcome the phone’s inertia. Stop pushing, and the phone immediately returns to (apparent) rest once again. One would expect this inertia and friction to keep it in place until an outside force (e.g. a nudging finger) acted upon it — the phone weighs about 4.9 ounces (140 grammes), and the leather of the arm is matte, not shiny.

I tried a simple experiment: with the phone at rest on the arm, I lightly rested my index finger on one side of it and my thumb on the other (being careful not to press down on the leather of the arm), and left a small distance between finger/thumb and phone. I waited, making sure I sat completely still so as not to cause any vibrations through the sofa. After a few minutes, I felt the phone gently nudging my thumb — it had begun to move, seemingly of its own accord. I did this a few times, and proved that even if the phone is placed only slightly off-centre on the arm (and I do mean just a tiny amount), the inertia is slowly but surely overcome and the phone proceeds from what appears to be a standing start to gather momentum and slide down the arm, gathering speed until it falls off. Even when I thought the phone was dead centre on the arm (which, admittedly, is not flat but slightly curved, as you can see in the photo), if I waited long enough, it moved. It was as though a poltergeist was having fun with me.

I’ve concluded it’s a property of the aluminosilicate glass and the completely flat back of the iPhone 4S (the 3GS was curved — I don’t know if this would make a difference, but I don’t remember my 3GS behaving in this way). It’s so smooth, there are virtually no impurities on the surface area, or lumps and bumps, presumably even at the molecular level, to provide sufficient friction to help keep it in place.

Which, to my mind, begs the interesting question: when placed in such a position, is it always moving from the moment my fingers leave it supposedly at rest? Working backwards from the point of most movement, i.e. slipping down and off the arm, is there a point where it isn’t moving? (Halve the distance travelled from one measured point to the next, working backwards — does one ever arrive at point zero?) I’m no expert in physics, quantum or otherwise, but something in my hazy instinctual grasp of these things suggests to me there’s a strong possibility that my iPhone is constantly on the move no matter where I place it, and on whatever surface.

Well. Putting all the flakey physics to one side, it’s nice to finish with a bit of sound advice, so the upshot of this is: don’t leave your iPhone on the arm of your sofa (or anyone else’s) — especially if you’ve got laminate floors. iPhones are tough and can stand a clatter or two, but they’re not impervious to shock. Leave it somewhere where it can’t fall!