Thread: Japanese Razors

Newb; thanks for sharing your thoughts. A one who has considered getting a Japanese blade, it was interesting to hear your shaving comparisons.

Originally Posted by Newbflat

I feel that scales are a bit of an affectation brought about by the need to have an attached edge cover.... kind of the unspoken flaw in the system.... IMHO.

This has come up before. In another thread, a member (I forget who, but I think it was one of our resident bladesmiths) mentioned that the scales on western razors help maintain the angle of the blade. As a physicist, I believe I can actually explain why.

There are many parallels between spinning objects and objects moving in straight line motion. This is useful, because straight line motion is easy to understand. As you probably know, in straight line motion, F=ma, where F is the force, m is the mass, and a is the acceleration. Let's reorganize that to a=F/m. Essentially, an object with a small mass will be accelerated rapidly by small forces. If you increase the mass, you will need more force to achieve the same acceleration.

The parallel equation for rotation is t=Ia (the t is supposed to be a tau and the a is supposed to be an alpha). It can be rewritten as a=t/I. Here, t is the torque (which you are probably familiar with), a is the angular acceleration (rate of change of spin), and I is the moment of inertia. I is the analog to mass, in that an object with a larger I requires more torque to achieve a given angular acceleration than an object with a small I.

So what do scales have to do with I. This comes out of the definition of I, but, in short, an object with mass distributed far away from it's axis of rotation has a larger I. So, the blade alone, which your hands would tend to rotate around some line from the toe to the tail, would have a small I. Adding on scales adds mass away from the axis of rotation, thereby increasing the value of I. That means that applying a torque (which you do all the time with your hands) to a blade alone will result in a larger angular acceleration than if the blade had scales on it. What this means in practical terms is that a blade with scales is "easier to control" and "feels more stable."

So, the scales do have a functional purpose. Not to say that many people couldn't control the blade without scales; they just make it easier.
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In my high school AP physics class, when we learned about torque and moment of inertia, we had a fun problem presented to us. Will a short rod or a long rod be easier to balance upright on the palm of your hand? I'm sure we all knew the answer from practical experience, but we were caught up in the "book physics." I myself said a short rod, because, at a given angle, the torque will be less than the long rod (because of the perpendicular distance from the base of the rod to it's center of mass, where the force of gravity is applied for the sake of simplicity). My reasoning was true, but I failed to consider the value of I. A longer rod has a much larger I than a shorter rod, so it's angular acceleration is very low. This makes it easy to move your hand (and the base of the rod) to keep the rod upright. To point out our silly mistakes, the teacher told us to balance our pencils upright on our hands while he balanced a meter stick on his.

Remember folks, fiziks is fun!!