In today’s Academic Minute, Dr. Mike Wheatland of the University of Sydney explains the gravity-defying physics of a falling Slinky.
Mike Wheatland is an associate professor of physics at the University of Sydney. His research interests include solar flares and solar activity, the statistics of that activity, coronal magnetic fields, solar-terrestrial relations, and Bayesian probability. He has also worked on a project to model the physics of a falling slinky.
Dr. Mike Wheatland – Physics of a Falling Slinky
The slinky is a child's toy, but it fascinates adults as well. Recently, with a colleague, I investigated the slinky version of the Indian rope trick. If you let a slinky hang under gravity suspended at its top end and then release the top, the bottom of the slinky does not begin to fall for about a quarter of a second, at which time the falling top section collides with the bottom. The bottom of the slinky hangs in mid air as if by magic.
The explanation is that the tension in the slinky, which supports the mass below, relaxes from the top downwards after the release of the top end, as the turns of the slinky collapse one by one. A tension wave runs down the slinky releasing the tension, and ahead of a wave front the turns are still stretched, and so support the mass below. The bottom of the slinky does not know that the top has been released until the wave front arrives.
With Rod Cross, another physicist, I investigated the falling slinky. Rod filmed the process and extracted data for the position of the top of the slinky versus time. I modeled this based on Newton's second law of motion. I used an existing model from the literature, but improved the description of the way that the slinky turns collapse. A paper on the work was recently published in the American Journal of Physics.
To me, the message of the falling slinky is that everyday objects sometimes exhibit unusual physics. Newton's laws of motion are more than 300 years old, but they can still surprise.