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Tacky – adding science to a sticky situation.

Posted by Mason Dannatt on

Tacky – adding science to a sticky situation.

You don’t have to be Issac Newton to understand the empirical results of your last stone loading session.

You either picked up the stone or you didn’t.

You put sticky stuff on your arms. Then you either picked it up or you didn’t.

If you are happy to leave it there that’s fine. But if you want to take a step out of the cave and look a little deeper into what’s ACTAULLY happening in those 20 inches from the floor to your knees, read on, and unlock the secrets of friction, and the role it plays on loading atlas stones.

Stone lifting 101 always states we are trying to lift a large, round object. It doesn’t have a bar or a handle to hang onto, so we are relying on friction (between our skin and the stone) to allow us to impart enough upwards force to defy gravity and get the stone to our knees. Simple, right? Unfortunately, to accurately quantify this situation, the free body diagram (the thing physicists use to work stuff out like this) isn’t as simple as the textbook example of a block sliding down an incline. Below is a hand drawn one I am working with.

 

                             

The fundamental law for dry static frictions suggests –

So, to explain, dry friction suggests the skin is dry, the concrete is dry. Static friction suggests that you haven’t lost grip on the stone, it’s not sliding in your hands. To calculate kinetic friction we need another whole equation.

Basically, the equation suggests that if the number on the left (the frictional force) is  equal or smaller than the number on the right (the force you are applying to the stone), multiplied by the coefficient of friction; the stone should stay in your arms.

Actually working out these numbers would be very challenging. For example, there are multiple angles of force you are applying to the stone (forearm, wrist, fingertips), is your left generating as much as your right, is the coefficient of friction the same on all surfaces?

What we can take away from it is this. The coefficient of friction between your skin and the concrete is critical to the equation. I can’t locate an actual one for human skin and concrete, but here is two examples. Coeff between concrete and wood = 0.62, coeff between concrete and rubber = 1. What that means is, very simply, less force is required to resist gravity if rubber is in contact with concrete, compared with wood. I would imagine skin has a coeff somewhere between the two, 0.7 perhaps?

This is based on the surfaces involved, and the contact area of the two. The below picture is a macroscopic view of two rough surfaces.

                    

Let’s get a bit more specific to stone lifting though. Below is a hand drawn diagram of two stones, at the macroscopic level.

                     

Although I have drawn the skin as flat, in the real world what happens is, on the left, the rough sharp edges of the stone will bite in to your skin and create a higher coefficient of friction than the smooth stone on the right, hence needing less normal force to ‘grip’ the stone.

Now let’s add tacky.

 

                 

Now we see the layer of tacky on the left has less impact, as it is held away from the full surface of the stone by the fact it is so rough. On the right, the layer of tacky is able to create a significantly better percentage of contact with both skin and smooth stone, greatly increasing the coefficient of friction.

Science says – No tacky; rough stone is better. Tacky; smooth stone is better.

Note – Despite it being a by product of the casting process, SorS intentionally leaves one side of their stones slightly rougher than the other, so you get the best of both worlds!

So we have established that tacky is good, but how much tacky is too much tacky?

Well, firstly and simply, the more tacky you get on the stone, the more will end up in your lap, on your tee shirt and on the floor. All the extra grip is now going to be working against you, adding to the force required to lift the stone.

But secondly, at some point, when the layer of tacky becomes thick enough, it will begin to act like a lubricant rather than an adhesive. Check out the graphic below.

                                  

When the lifting force you are applying becomes significant enough, the actual bonds between the tacky will break, and it will slide over itself.  Just imagine you had a 30mm thick coating of tacky on your arms, as an extreme example.

In summary; if the stone you are attempting has a smooth even surface, a very thin, even layer of tacky on your arms will provide you with the best result. If the stones is rough, you can attempt to fill all the little holes in with tacky, but at some stage it is going to start working against you, so again, less is more!


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