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
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
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 firstname.lastname@example.org.
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