Special Physics Photo of the Week

Physics Photo of the Week     

December 9, 2022

Slinky Defies Gravity

My grandson Jay is holding a yellow plastic "Slinky" at one end is about to drop it.  We recorded a slow-motion video for the falling slinky.  When you watch the video (see link below) be sure to pay attention to the bottom of the yellow slinky - it seems to defy gravity and doesn't fall until the rest of the slinky "catches" it and the collapsed slinky continues is descent to the floor.  Shouldn't the whole slinky fall at once?  The bottom edge of the stretched-out slinky simply "floats" at the level just below Jay's knee while the top of the slinky falls due to gravity.   

Press this link to see the slow motion video.

Why does the base of the slinky remain suspended motionless while the top of the spring falls rapidly?  Shouldn't the whole stretched out slinky fall at once?

The answer lies in the fact that gravity is not the only force acting on each of the coils of the slinky spring.  The slinky is stretched out by the force of it's own weight pulling down on it and stretching it.  At the same time there is an internal force from the stretched-out spring coils pulling up on each of the lower coils.  (If the upwards internal force were missing, the weight would cause the coils to fall down).  While Jay holds the top coil of the slinky, the whole spring is suspended in a stationary equilibrium.  There is no falling, the spring is essentially motionless.  Gravity pulling down each coil is balanced by the internal force due to the spring's stretch that pulls up on each coil.  As soon as Jay releases the top of the spring, the top coils of the spring begin to fall.  The top of the spring is pulled down by both gravity and the internal force caused by the spring's stretch.   At the same time that the top of the spring starts to fall, the bottom of the spring is still stretched.  The internal stretch force pulling up on the bottom coils continues to cancel the downward gravity on the bottom coils.  Thus the bottom coils do not begin to fall until the "wave" of the collapsing spring reaches the bottom of the coil.

The top coils of the spring in the free fall actually fall faster than the acceleration due to gravity because the internal forces from the spring's stretch are pulling down on the top coils in addition to the force of gravity.  When the spring is completely collapsed the collapsed mass of the spring as a whole continues its downward motion with the acceleration due to gravity.  The center of mass of the collapsing slinky - approximately the middle portion of the slinky - would fall with the acceleration due to gravity.  The top of the spring falls further in the same time.  The bottom of the spring falls only a small amount.  But all portions of the spring hit the floor at the same time.

Physics students, and anyone else, may wish to make a similar video - many smart phones have a slow-motion capability in their video photo photography in order to repeat the experiment.  The simplest experiment would be to video record the falling slinky along with a fairly solid ball (baseball, softball, or a golf ball.  Not a pingpong or whiffle ball).  Look at your video.  You should see that the top of the slinky will fall faster (due to the collapsing nature of the spring) than the solid ball.  A number of other experiments could be done with a video-equipped smart phone along with a free video analysis program (Tracker, developed and written by Douglas Brown - https://physlets.org/tracker/ )

In the video featured above, Jay has also released a traditional metal slinky at the same time as the plastic slinky, but the metal slinky is very difficult to see in the video due to the lack of a contrasting color with the background.  The metal slinky has a stronger elastic stretch effect, so the top of the metal slinky falls with a larger acceleration than the than that of the plastic slinky.