5/26/2023 0 Comments Center of gravity lab rodTiny mysteries and everyday wonders abound, just waiting to be discovered and described by an inquiring mind - and that’s part of what makes science so great! It’s wonderful that science - the same kind of science that enables us to build durable bridges and lightweight airplanes, to loft satellites to the edge of the solar system and to explore the deepest ocean valleys - enabled these researchers to predict and confirm such an odd, counter-intuitive result in something so seemingly mundane and straightforward. I often end posts by marvelling at the vast richness of the world, but this time its our ability to persistently question and precisely conceive the world that amazes me. Given that we can measure and understand fluctuations in the universe’s infancy, it’s pretty striking that we’re still learning new things about something as familiar as a falling chain. Impact with a table doesn’t speed up an ordinary link chain, since the last link decouples from the rest of the chain (click picture for video) ![]() The free end speeds up for the same reason that you turn quickly if you grab onto something while running or skating: conservation of angular momentum. Since the rod is at an angle, one end will strike the floor first the other end of the rod will pivot around the contact point with the ground, making it fall faster. How would such a mechanism actually work in practice? The team came up with several different designs where the last link could pull on the remainder of the falling chain The most straightforward one is based on the idea of a tilted rod hitting the floor. Depending on the strength and direction of that force, the last link might push back against the remaining length of chain and slow it down or it might pull on the chain, speeding its fall. If the last link in the chain continued to interact with the link above it, it could exert a force on the falling portion of the chain. The team weren’t convinced that it would always hold, though, so they decided to work out the mechanics of what would happen if the assumption wasn’t true. It’s a reasonable assumption, given how loosely links are connected and the way chains generally behave. ![]() ![]() The standard solution assumes that the last link in the falling chain (shown in red in the picture) effectively disconnects from the link above when it hits the pile - in other words, that it stops without affecting the rest of the chain. The two strange-looking chains were dropped from the same height at the same time, but the one on the left, which fell into a pile on a table, fell faster than an identical chain falling past the table.Īnoop Grewal, Phillip Johnson, and Andy Ruina made their discovery by taking a second look at a classic physics textbook problem involving a chain falling into a pile. As this picture from their experiment shows, they were right. By carefully studying its mechanics, they showed how the impact could actually pull the remainder of the chain downwards. In 2011, a team of physicists at Cornell University in Ithaca, New York showed that when a falling chain hits something (say, a table), it might, contrary to all intuition, speed up and fall faster than it would if it fell freely.
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