Sunday, May 4, 2014

Phasinating Physics: Friction.

There are many types of friction, like dry friction or fluid friction etc. but I intend talking about friction here in a general sense.



Friction affects us all the time and is hugely important to our everyday lives. It permeates our day to day experiences so thoroughly that we have real difficulty imagining scenarios where there isn’t any. This is most obvious when we think about what happens in space. When describing how objects in space behave, most people have trouble really getting it, even after having it explained. This is no great failing on their part. It is completely alien to our day to day experience. There is a scene in the recent movie Gravity which most people familiar enough with physics will find particularly bizarre and nonsensical. People less familiar with physics might not notice at all what is a rather glaring departure from the normal functioning of our universe.

Without friction we wouldn’t be able to move about, at least not anything like as easily as we do. When you walk, you place a foot on the ground and them you essentially push it backwards. Trying this on ice, where there is less friction, you might find that your foot does move backwards and you may be deposited onto the ice face-first. On ground where there is more traction (which is directly related to friction), the result of your attempt to push your leg backward is to actually push your body forward. This is thanks to the equal and opposite force applied to you. You don’t move (technically you don’t change your state of motion) unless an external force pushes you - Newton’s first law. It is difficult to grasp at first but an equal but opposite force is being applied to you by the ground - Newton’s third law. The earth is pushing you forward.

Imagine you are floating next to the ISS or some other object in space; you are stationary relative to it. You are holding a basketball. If you throw the ball away from you, you will notice (by paying attention to your motion relative to the ISS) that you start to move in the opposite direction that you threw the ball. When you exerted a force on the ball, it exerted an equal and opposite force on you. The harder you threw the ball, the faster you will be moving. You will be moving slower than the ball but this is due to the fact that you weigh more. If you pushed against an object that was exactly the same weight as you, you would move away from the point you pushed it, at the exact same speed as it moves away from the same point (but in the opposite direction). No matter how light the object that you throw, you will move away from it in space and all other things being equal, you would stay in motion forever. If you fired a bullet from a gun in space, not only could you direct yourself about by successive firings (this is basically how reaction engines - like rocket engines - work) but the bullet would continue on forever until it hits something. This concept was rather amusingly referred to in one of the mass effect video games. This conversation is overheard by the protagonist:

Gunnery Chief: This, recruits, is a 20-kilo ferrous slug. Feel the weight. Every five seconds, the main gun of an Everest-class dreadnought accelerates one to 1.3 percent of light speed. It impacts with the force of a 38-kilotomb bomb. That is three times the yield of the city buster dropped on Hiroshima back on Earth. That means Sir Isaac Newton is the deadliest son-of-a-bitch in space. Now! Serviceman Burnside! What is Newton's First Law?

First Recruit: Sir! A object in motion stays in motion, sir!

Gunnery Chief: No credit for partial answers, maggot!

First Recruit: Sir! Unless acted on by an outside force, sir!

Gunnery Chief: Damn straight! I dare to assume you ignorant jackasses know that space is empty. Once you fire this husk of metal, it keeps going till it hits something. That can be a ship, or the planet behind that ship. It might go off into deep space and hit somebody else in ten thousand years. If you pull the trigger on this, you're ruining someone's day, somewhere and sometime. That is why you check your damn targets! That is why you wait for the computer to give you a damn firing solution! That is why, Serviceman Chung, we do not "eyeball it!" This is a weapon of mass destruction. You are not a cowboy shooting from the hip!

Second Recruit: Sir, yes sir! 


When you are on earth, the same forces are at work; the difference is friction. That is what stops you from floating backwards until you hit something whenever you throw a ball. It is worth noting that when you walk forwards, you are pushing the earth backwards. Imperceptibly but you are actually doing so. The Chuck Norris joke “When Chuck Norris does pushups, he isn’t pushing himself up he is pushing the earth down”; it is actually true that Chuck is pushing the earth down (ignoring the possible niggles about up and down in this context) albeit to an impossibly negligible extent.

Friction is everywhere in our lives and yet it is still problematic from a scientific point of view. The field of physics that tries to understand friction is tribology.There are many factors that contribute to friction. Surface irregularities, chemical properties, physical properties and so on all affect friction. Understanding exactly what interplay is at work is difficult and ongoing. When you put your cup of coffee down on your desk, friction is the reason is doesn’t go sliding around the table top, same when you take your hand off your mouse.

Friction can convert kinetic energy into heat, in some cases rather spectacularly. Re-entering spacecraft would often be cited as an example of this but a lot of that heat is generated by the extreme and sudden compression that the air in front of the spacecraft experiences. Some of it is friction. Anything moving that fast through a medium, even the relatively rarefied upper atmosphere is going to generate heat through friction. You can start fires with some sticks and friction. With some notable exceptions, like carpet burn, “road rash” and fiery atmospheric death, friction is generally your friend!



For fun, try imagining what would happen in the following scenario: You are in a perfectly frictionless environment. There is an upright car tyre rotating counter-clockwise very fast (say 1000 rpm – equivalent to roughly 70mph for a 15” tyre). It is dropped onto a surface. What direction does it bounce?

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