WHY A DOOR SLAMS.

Mataura Ensign, 22 April 1913, Page 5

WHY A DOOR SLAMS.

All have wondered, at some time «r other, why a door, opetu*d nearly to th* limit, will close when there* is a draught of wind going through tho opening. Certainly the wind does not get in behind the door and shove it; for the draught is through the opening. , The explanation of this fact (says an American writer) lies in a fundamental principle of moving fluids, and is tho saiho principle that makes a basel:alj curve. Whenever there is a current in the air the sidewise pressure is least in that current; so when the draught blows throng!) the opening to the door, the pressure on thai side of the door is decreased, and the pressure on the other side will start slowly to move tin' door, making it go faster and faster, till the door swings into tbn draught itself and starts going in th« direction of the draught. When it does go in this direction, of course th» draught helps it along, because it is now not the sidewise motion that counts, but the forward motion. When a ball is rotating in the air it carries around with it a good deal of air sticking to the sides. This, then, is the same tiling as saying that a current of air is Hewing round the rotating ball in the dirootion of rotation. Now, if a wind blow* against that ball, it is easy to see that the little rotating current will be with the wind on one side and against it on the other, so that on one side the velocity of the air will b«* increased and on the other diminished. So thegreatei pressure on the slow side will shove the ball against the low pressure on tho | fast side. Now, throwing a ball in the | air is the same as making a wind blow against it; so if a bail lie thrown and rotated at the same time it will necesi sarily curve.