麻雀： 壁を作る

Because applying Science to mundane things keeps me sane.

Most people who play Mahjong (casually of course) lack the ability to properly lift one wall on top of the other. It isn't a fundamental skill to know, but it can speed up set up.

Overall it isn't a difficult process. First assemble your two walls of 17 tiles (18 in Hong Kong Style of course).

After that, either push the front wall forward, or the back wall backward; this leaves enough space for your hands. In this case I push the front wall forward, but on Mahjong tables, it is much better to push both walls backward, and then push the front wall forward, because there is a "frame" on the table that helps you line up the tiles.

Grasp each end of the back wall like so:

Depending on how large the tiles are, the number of tiles you can touch with your fingers vary. Since my tiles are larger than normal, I can only touch 4 tiles. It is important that you touch as many tiles as possible, because it makes it easier to balance the wall.

You must grasp the wall in such a way so that the wall remains in static equilibrium. If we draw a force balance diagram:

Each arrow's size is the relative strength that each part of the hand exerts on the wall. You can see that not only is each hand pushing horizontally, but the fingers of each hand are pushing vertically (This is in a 2D plane).

Once in equilibrium, you can then lift the wall vertically upward (the z-axis, since this is now in 3D), while maintaining equilibrium on the x and y axes. If equilibrium is not maintained in the 2D plane, the wall you are holding will most likely collapse. Of course since Microsoft Paint is incapable of 3D diagramming (for obvious reasons), I can't really draw a force diagram. But all that's really going on is that gravity is pushing downward, whereas your hands exert a force greater than or equal to gravity to prevent the wall from dropping.

Here is an bottom-up view:

Finally, you take the wall and place it on top of the wall that remained stationary. Do this either one direction at a time (as most beginners will start to) or through an arc motion (When maintaining the wall becomes more familiar to you). The whole process of lifting a wall should take a second; two seconds at most. Taking any longer will tire your hands, as you are opposing gravity, in addition to the low coefficient of static friction between the tiles, so it's common for tiles to "droop" if you aren't quick enough.

Overall it's a cool trick that speeds up set up, so although you don't have to learn it, you should if you frequently play Mahjong.