Friction, traction, torque - oh my...

[Lloyd Burns]

Friction, Traction, & Torque

Friction comes in many forms: static, sliding, and rollling are but a few. Sometimes we call an energy loss 'friction' rather tham explain exactly what it is, because the loss acts just like a friction. Remember that friction is a human construct to make it easier to explain Nature - it probably doesn't work that way at all. The 1940's cartoon joke about "aerodynamically, a bee cannot fly - but the bees never read the book, and they just go on flying" really says that the jokester's aerodynamics are woefully inadequate for the study of bee flight.

Static friction is sometimes thought of as the microscopic mountains of roughness on one surface fitting into the microscopic valleys of roughness on another surface, locking the two together. Often we get more friction by roughing up the surfaces - but not for the shiny surface of leather belts turning smooth drive pulleys on lathes and other factory machines in the days when electric motors were really costly (the days of the steam engine in Dean's 'house').

Sliding friction could be envisioned as the two surfaces bouncing along, the peaks of their microscopic mountains banging into one another as the peaks of the moving surfaces fly over the valleys. We could then evision putting something (like oil) between the surfaces to float the surfaces away from one another to lower the friction.

Static friction, by definition only occuring when the one surface does not move relative to the other, is often more than the friction when the same surfaces slide (move past each other). A rolling wheel's point of contact is stationary on the winter road, until the brakes siezes it, and prevents turning: then it slides over the road. A bookcase, pushed to one side by little brother to re-arrange the furniture, resists movement (static friction with the floor is high). Little brother's shoes slip on the floor: the friction he can muster is not equal to the friction between the case and the floor. Big Brothe comes along, and with the extra weight on his shoes helping F=uN to make more friction, his shoes do not slip (static). The bookcase resists (static), then with a bit of a bang, starts to slide (!). Big brother now doesn't need to push as hard to keep the case sliding.

Rolling friction is a funny kind of friction: It is due to a bike wheel of a railway wheel having to become flat under the load it carries. Since it would be round if it carried no load (bike on a rack), this means it had to change shape (deform). The energy to change shape had to come from somewhere. It is easy to calculate the energy loss if you conjure it as a friction; hard, if you try to examine the dynamic plastic deformation of the rail wheel.

Torque is a force acting in a circle, or turning, often within a shaft. A given amount of torque is constant whether it exerts a large force tangentially, close to the centre of rotation, or a small force farther out. (T = F(tangential) x radius) Hold a weight at the end of your arm, and think of the arm being 'torqued' around your shoulder. The weight is pulled down, at right angles to your arm's length, that is, tangential to the circle your hand goes in around your shoulder. More weight requires more pull from your muscles; different muscles if somebody tries to lift the weight and you resist. If you notice, this example uses forces (gravity, muscles) which are not torques, but because their lines of action do not not go through the pivot point (shoulder) they generate torques to 'spin' the arm one way or the other. A motor continuously generates a tangential pull around the armature, a short distance away from the axis of the armature's (rotor's) turning: what you'd observe in the shaft is a turning force, a torque.

If you turn a shaft with the torque, and put a wheel on the shaft, and the wheel has friction with a surface, the torque of the shaft will turn the wheel (or not) and a force of the wheel pushing the surface (and vice versa) will be observed. A small diameter wheel exerts a large force at its circumference, a larger wheel (or gear) exerts a smaller force as the radius increases. (F = T / radius)

Traction depends on the friction between the surfaces. The push depends on the torque delivered to the levers that push the one surface past the other. If your robot has adequate static friction, and adequate torque, it will move in the desired direction. If it has insuffienct friction with the floor, it will slide.

-=-=-=-=-=-=-=-=-=-=-=-=

BTW, on a macroscopic level, sand is a surface (try jumping of a cliff onto a beach) and so is the carpet. Each can be examined microscopically or treated as a surface.

Don't confuse reality with human tricks to make things easier to work with. The physics of Aristotle asked why a thrown javelin keeps moving: they figured all motion that wasn't the object returning to its 'natural' place (where its nature demanded it should be) was "Violent" motion, which, by definition, required a force to continue. The javelin was not in "Natural" motion, so a force must exist to keep it going horizontally. They invented a force to keep it going (the air molecules, closing in around the end of the javelin, to avoid a vacuum [the 'void'] banged into the end of the javelin, propelling it forward. Following this idea, Boeing should make the aft end of their fuselages come not to a point, but a nice flat circle.

On the other hand, Newton and Gallileo saw that a moving object will remain moving in a straight line unless a force prevents this. They had to invent something to slow things, and turn things. They invented what we call friction. We use rubber tires for more of it, and oil for less of it. We use the friction idea because it works better than the violent motion option. Being pragmatists, we don't care if it's true, if it works.

Maybe you will create the next construct - maybe we'll all talk of demons grabbing things, and of oil offered as a sacrifice to make the demons go away.

Sorry this is so long, I hope it helps.