Where the shape comes from
An arrow standing point upward: the head at the top, the shaft down the middle, the notch and the flights at the foot. An arrow flies straight and lands where it was aimed, so the radical carries accuracy as well as archery: 知 words that go straight to the mark, 短 measured against a standard length, 医 the quiver, 矮. Do not read it as the grain plant 禾, which has a drooping head instead of a point.