Posts Tagged‘friction’

Struggling Against Static Electricity.

Everyone knows the standard static electricity experiment. You grab yourself a perspex rod and a wool cloth and, after some vigorous rubbing (often with a few innuendos thrown in), suddenly your perspex has the ability to attract pieces of paper. Most people will also understand the mechanism of action, the transference of charge that leaves the rod negatively charged and the cloth positively charged. What most people won’t know however is that friction isn’t required to generate a static charge. This is what can lead to hilarious situations like the one in the video below:

So if friction isn’t a requirement for generating a static charge how do these address labels get it? The answer is actually pretty interesting and has to do with the way adhesives work. For these address labels the adhesion comes from a chemical reaction, meaning that the address labels had a form of bond with the backing before they were torn apart. When this bond is broken both materials will gain or lose electrons, depending on where the material sits on the triboelectric series. I’d hazard a guess that the material that the address labels is made up of tends more towards the negative end of the spectrum, meaning that bin holds a strong negative charge.

This is what is responsible for the labels floating around in a seemingly random fashion before ejecting themselves out onto the floor. The effect wouldn’t have been immediate, each label would only carry a small negative charge, however past a certain point the negative field would have become big enough to repulse the small weight of each of the labels. If they were so inclined they could throw a positively charged piece of plastic in there and they’d all be attracted which would also be pretty interesting.

Or, if you wanted some real fun, if they rubbed their head with a balloon and then dunked themselves in there all the labels would gleefully stick to them. Not that that proves much, just that it’d be hilarious to see someone with shipping label backing stuck to them.

Euler’s Disk: Surprising Complexity.

As long time readers will know I’m a fan of simple experiments or demonstrations that have some underpinning scientific phenomenon. It was things like these that first spurred my interest in science, especially since places like Questacon (a must visit place if you ever find yourself in Canberra) were filled to the brim with experiments like them. Thus whenever I find one I feel compelled to share it, not so much for myself but in the hopes that when someone sees it their curiosity will be piqued and they’ll pass that same passion onto others. In that vein I give you Euler’s Disk, one of the most fascinating science based toys I’ve come across:

The disk gets its name from Leonhard Euler, an eighteenth century physicist and mathematician who was behind such revelations as infinitesimal calculus and many other fundamental things. He studied the disk as part of his other research however it wasn’t until recently that they found themselves back in the limelight again. Back in 2000 Cambridge researcher Keith Moffatt demonstrated that air resistance played only a small part in the rate in which the disk slowed down with the vast majority coming from the rolling resistance between the surface and the disk’s edge.

What interest me about it most is the gradual speed up of the revolutions coupled with the increasingly bizarre noise that accompanies it. Then, right at the end when it appears to be spinning at its fastest the disk stops, as if some outside force robbed it of all its momentum instantly. This demonstrates how momentum is conserved as the rate of precession of the disk increases as it spins downward. Explaining the phenomenon though is much harder than just watching it however, which is why it’s such a great scientific toy.

Friction: A Force to be Reckoned With.

Kinematics was my least favourite part of physics, mostly because I always had a rough time wrapping my head around the various rules and principles that govern the way things move in our world. However one lesson always stuck with me in my head, the one relating to friction and it’s various forms. Whilst I’m sure the teacher delighted in tricking us all by asking us what kind of friction a rolling tire has (hint: it’s either static or kinetic and it’s not the one you’d first think it is) that example rooted the principle firmly in my head. Understanding that made further concepts a lot easier to grasp although I’d never really considered friction a powerful force until I saw this:

What you’re seeing happen here is a process called Friction Welding although in technical terms it’s actually not welding at all. Instead it’s actually a type of forging as in traditional welding two pieces of metal are joined via melting whereas in friction welding no such melt occurs. This process has a lot of advantages most notably allowing 2 dissimilar metals, say high grade aluminium and steel (a common pair in space fairing missions), to be joined together. Doing this process via other means is extremely difficult due to the different melting points of each material and would likely lead to a much weaker bond. Friction welding by comparison always creates a full strength bond without the additional weight introduced via other methods.

Interestingly enough this process can also be used with materials other than metals, specifically thermoplastics which are a type of plastic that becomes pliable under heat. Friction welding can then also be used to join said plastics onto metal surfaces, enabling cross material bonds that are far stronger than those  that could be achieved via other methods.

Pretty fascinating, isn’t it?

A Rather Novel Way Of Starting A Fire.

If you trace back along the path of human evolution (the homo genus to be more specific) there’s a period where our species started to undergo rapid changes. The actual time varies wildly depending on the sources you read but the cause isn’t: it was when we learnt to control fire. Fire enabled our ancestors to do many things that simply weren’t possible before like cooking food (which provides easier access to calories and nutrition), doing activities at night as well as during the day and even protecting themselves from animals and insects. Indeed the species were are today, one that is well adapted for cooked food, is because of our beginnings as masters of fire.

I’m also somewhat fascinated with the creation of fire, possibly from a purely primal level, but also because there’s numerous different ways to do it and each of them exploit a physical principle. One of the most interesting ones I saw recently was someone using a hammer to light a cigarette:

At the highest level what is being done here is that kinetic energy, from the hammer falling on the piece of metal, is being translated into heat. This is accomplished by the bending and warping of the metal that occurs when its struck by the hammer which breaks down the bonds between the metal atoms causing them to release heat. The second part of the trick here is that they then utilize a highly flammable tinder, I.E. the cigarette, which has a flash point below that of the temperature of the metal. Drawing air over it provides more oxygen and with that you have all the ingredients you need for fire.

Of course it’s not the most practical way of creating fire given the materials required to do it. You’re much better off with a flint and steel as they produce sparks with temperatures that far exceed that of hammered metal. That is of course if you don’t have any matches, cigarette lighters or any number of modern fire making devices handy but for pure reliability you really can’t go past a good old fashioned piece of flint and steel.