1177. Why are sparks generated when iron is brought in contact with a spinning grinding wheel? — JF, Rochester, NY
When the iron touches the spinning wheel, the two experience sliding or "dynamic" friction—the iron acts to slow the wheel while the wheel acts to move the iron. Because you hold the iron in place, it doesn't move but its surface begins to experience severe wear—the iron is skidding across the surface of the wheel. Sharp projections from the wheel are tearing particles away from the iron and throwing them in the direction of the wheel surface's motion. Because the two surfaces, iron and wheel, are pushing on one another and they are moving relative to one another in the directions of their forces, they are doing physical work on one another—meaning that they are exchanging energy. This energy is actually being converted from the wheel's rotational energy into thermal energy in the iron and in the wheel, both of which become hot. You can feel similar heating by rubbing you hands against one another vigorously. The wheel's surface begins to glow red-hot and the particles that fly off the iron emerge so hot that they burn in the air. The sparks you see are the iron particles burning up. Depending on what type of iron or steel you use, you'll see different spark patterns. An expert can actually identify an alloy by this pattern.