Physics Photo of the Week

March 30, 2012

Defiance of Gravity!

This is a gravity experiment similar to Galileo's famous Tower of Pisa experiment where he demonstrated that all objects fall at the same acceleration of gravity regardless of their mass in the absence of friction.  See the PPOW for March 2, 2012.

However, this experiment appears to defy gravity.  A "slinky" spring is held at one end and a roll of masking tape is fastened to the free end.  The top end consists of a tied-up bunch of slinky coils that acts as a falling mass.  In the experimenter's other hand is an ordinary golf ball that is dropped simultaneously with the slinky.  One would think that the whole stretched-out slinky would fall as a single unit as soon as released and that the mass at the lower end would begin to fall as soon as the upper end was released.

But no!  Watch the short video clip carefully.  Both the slinky and the golf ball are released simultaneously, but the lower end of the slinky hovers motionless while the top of the slinky falls faster than gravity's action on the golf ball!  The bottom of the slinky levitates while the top of the slinky is falling.  The bottom of the slinky appears to defy gravity and the top of the slinky appears to violate Galileo's Pisa experiment by falling faster than the golf ball.

The laws of physics are not violated here.  The secret to understanding is to realize that the stretch of the spring exerts an upward force on the lower weight.  Before the spring is released, the weight of the lower mass stretches the spring.  Gravity pulls on the lower mass.  The stretch of the spring pulls up equal and opposite on the lower mass.  The result is zero force on the lower mass.  When the top of the spring is released, the spring is still stretched.  The stretching force continues to pull up on the lower mass equal to the lower mass's weight.  Gravity and the stretch of the spring continue to cancel each other and the lower weight remains suspended. 

The top of the spring exhibits different behavior when released.  In addition to the downward force of gravity on the top of the spring, the stretch of the spring also pulls down on the top of the spring.  As a result both the spring's stretch and gravity are pulling down on the top of the spring.  Notice that the top falls faster than the golf ball.  The total forces on the bottom of the spring cancel and the bottom of the spring does not begin to fall until the spring collapses.

When the slinky collapses, it collapses as a wave.  The bottom remains stretched, exerting is upward force on the lower weight keeping it levitated.  The levitation is maintained until the collapsing wave reaches the bottom end of the slinky.

I am indebted to Dr. Stephen Cartier of Warren Wilson College Chemistry Department for sending me a link of a YouTube video (http://www.youtube.com/watch?v=eCMmmEEyOO0) featuring Professor W. G. Unruh of the University of British Columbia demonstrating this effect along with his excellent explanation.  Dr. Victoria Collins ran the video camera while yours truly dropped the slinky.


Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

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