Is the gravity still turned on? A free falling Slinky appears to defy the laws of physics. Or does it? Credit: You Tube.
This video (about 6 minutes) of a free falling Slinky shot with a high-speed camera makes it look like the Slinky is floating in air before falling. In the first minute of the video, a Slinky is dropped from the top of a building and seems to levitate as we watch its fall in slow motion.
The filming team had the good idea to drop it from a building where the windows create a background grid. It’s amazing to watch the Slinky fall, while the bottom does not move with respect to the “grid.”
University of Sydney (Australia) associate professor Mike Wheatland of explains that that the tension in the Slinky causes the top of the Slinky “to snap down,” giving it the levitating illusion. Wheatland has done some modeling of the Slinky’s motion to explain what’s going on, which he demonstrates in the video. He says it takes only about one third of a second for the Slinky to collapse and continue its fall.
Is there any “real world” reason to understand how Slinkys fall? According to Wheatland there is.
“What you’re doing is changing something at the top, and then there is a finite time for that information about the change to get to the bottom of the Slinky. …That happens even with a rigid bar, like a steel bar, it’s just that the time is very, very short,” he says.
Asked to explain what he means about “information,”—to distinguish it from the ubiquitous use of “information,” for example, on the internet—he says “It’s a signal. Whenever you do something physically to effect a change, …you do something and there’s a cause and an effect, and between the two information has to propagate if they’re not at the same location physically.”
Hat tip to Kent Anderson at The Scholarly Kitchen.