G Force - Let's get the terminology right!

In this, the first of hopefully a long series of articles, let us take a closer look at this business of gravity.

The force due to gravity between two bodies is proportional to the mass of each, and inversely proportional to the square of the distance between them. So here we are on earth and the massive earth exerts a force on a particle, but equally that little particle exerts a force on the earth. The force is trying to accelerate the particle towards the centre of the earth and the particle is trying to accelerate the earth towards the centre of itself - earth wins!

Now, if that lowly particle approaches the size of the sun, the earth would go screaming off towards a roasting!
But hold on, there is a sun out there, so why aren't we toast? Well, because there are also suns on the other side that balance things out. Gravity wears off easily with distance, but there are enough suns with enough mass to make up for it.
Hmm, so everything just sits in the same place then?
Well, not quite. You see, the earth, the suns, the whole galactic mess is actually in motion with all kinds of gravitational forces pulling this way and that. The net result is that the net force acting on a particle results in a tendency to accelerate it in the direction from which the net force is coming. Because the mass of the earth is so much greater than that of any of the particles that we consider, we simplify the whole thing down and say that there is just the acceleration due to the earth and we call it "G".

So G is not a force at all, but acceleration. Boy, this is confusing.
The actual force that the earth exerts on the particle is given by the mass of the particle, M, multiplied by G, i.e. Force = MG

Fine so far, but everything we have looked at above is to do with the realm that we disparagingly refer to as "One G" - the natural, organic G that mineral processors were stuck with in the olden days. Enter the dizzy realms of enhanced, synthetic G and suddenly the earth is "bye the bye", still there, but of minor consequence. Although we loosely use the word "gravity" in this new domain, the fact is that it has nothing to do with gravity at all! This is all about force, centripetal force to be exact, and we get that by spinning the system around a centre of rotation.
No, no spelling mistake, the force is called centripetal.
The term centrifugal is borrowed from "centrifuge" and is not correct. The force is actually given by the mass multiplied by the angular velocity squared and the radius, F=mrw2 . The further the particle is from the centre, and the faster we spin it, the higher the force. If we generate a force of 60G, for example, that is equivalent to an earth 60 times our little planet's mass, or put another way, we have actually made that paltry little particle 60 times heavier! Wow! And it is this effect that we use to make the particle accelerate more aggressively in the slurry system to get those super separations you all demand.

Note that we have not used the word density once here. You see, gravity, whether au naturel or our artificial centripetal force doesn't care a jot about density, just mass. Now I can hear the process boys howling, but no, sorry, no density effect. Density, or SG, brings volume into the equation and that introduces us to an entirely different and counter-force called drag. It is to do with the surface and shape of the particle only and it is the reaction of the medium to the motion of the particle whizzing through it. Yes, and to do with the density and viscosity of the medium too, but lets save that for another day.

All clear as slurry? Good, because there are some subtleties in this simplistic lot that we will expand upon next time. And for those of you who were expecting sums and formulas. I promise I will get my bright colleagues to give you a few in future, but first lets get the terms right.