Final answer:
Iron cutting shears have a short cutting edge to exert higher pressure for cutting tougher materials, while cloth cutting scissors have a long cutting edge for smooth, consistent cuts on softer materials like fabric.
Step-by-step explanation:
The reasons iron cutting shears have a short cutting edge and cloth cutting scissors have a long cutting edge are mainly determined by the materials they are designed to cut and the mechanics involved in the cutting process. Iron cutting shears are used to cut through tougher materials, which requires them to apply a greater force concentrated over a smaller area to achieve the cut. The short cutting edge allows for greater pressure to be exerted with a given force, following the principle that pressure is force per unit area. Conversely, cloth cutting scissors have a long cutting edge because cloth is a softer material that often requires longer, smoother cuts. The longer edge allows for maintaining a consistent cut across a larger area, providing better control and efficiency when cutting large swaths of fabric.
Moreover, scissors operate as a double-lever system, with the handles acting as levers and the pivot point as the fulcrum. The difference in cutting edge length alters the leverage and mechanical advantage for the intended task. Short, powerful shears provide a greater mechanical advantage for cutting hard materials, while longer scissors allow for a longer motion, more suitable for cutting soft materials like cloth.