Friction fire lighting Having watched a few youtube guides on making a fire by friction, namely the spindle and fireboard method, I've heard many of the instructors talk about how the shape of the spindle at its ends should be point-like at the top and blunt/round at the bottom. They then go attribute this to the idea that one would want as little friction at the top and most friction at the bottom. Whilst this seems to be intuitively correct, my very basic knowledge of physics tells me that idealised friction is independent of apparent surface area and just proportional to the normal force (also proportional to actual contact area?). I tried thinking about this in terms of energy but still couldn't get my head around it. I also find it difficult to believe that the consensus of many, especially those with years of experience find this to be the case. It seems unlikely,though not impossible, that they have all come to wrong conclusion. So, my question is this: Is a blunt end for a spindle better than a sharp point, or is the shape irrelevant and if so how might you reconcile these 2 ideas (are they all wrong?)? Also if you have any empirical evidence to back this, or links to, that would be greatly appreciated.
 A: You want a point at top and a flat area at bottom.  You want most of the friction torque to be at the bottom where you are trying to create embers.
Given the same material, ideally the friction force is proportional to the normal force and the speed.  However, the friction torque of a rotating shaft is proportional to these times the radius.  It is this torque times the rotational speed that makes power, which causes the heat to make the embers.  Added to that, the friction is probably not that ideal and viscous.
What spins better on your table, a top or a coffee cup of the same weight?  Obviously the coffee cup stops spining much faster than the top.  Intuitively you can see that the coffee cup has much more friction against spinning than the top does.  This friction times rotation rate causes power to be dissipated as heat at the interface.
A: Well, I'm not sure if you can use the ideal friction for this case. But the surface area does matter, if you want to light something up.
Given the same normal force over 2 different surface areas, you would experience the same resistive force right? So given the same moving velocity (simplification), the power dissipation should be the same.
The question then simplifies to: Is it easier to set the material on fire to concentrate this power in a small area or let it spread over a larger area? I think the answer is obvious at this stage.
