Observations made in the independent slinky drops:
- Like the plastic slinky in the vertical relaxed drops, the bottom end of the slinky remains still when only one coil is held before dropping the slinky.
- A longitudinal wave that forms as the slinky collapses determines whether the bottom end of the slinky will remain still or move before the top end reaches it.
- This longitudinal wave forms faster and becomes more visible with increasing numbers of coils held before release.
- Up to a certain number of coils held (about 30, which comprise roughly 1/3 the mass of the slinky), the bottom end of the slinky remains motionless.
- Beyond that number, the longitudinal wave collides with the bottom before the top has a chance to overtake the wave, causing the bottom to move before the top reaches it.
- Unlike the plastic slinky in the vertical relaxed drops, it appears that the top end does not flip over the bottom end upon collapse. Instead, the top end merges with the bottom so that the slinky becomes fully contracted, and it keeps the same shape after collapse for the rest of its fall.
Observations made for baseball and slinky dropped together:
- The fewer the number of coils held before release, the faster the top end will accelerate, leaving the ball behind as it falls.
- The longitudinal wave forms in the same manner as in the independent metal slinky drops. The ball has no effect on the motion of the top end, primarily because the top end keeps little contact with the ball during the drops.
- The top end is clearly seen to fall faster than g.
Observations made during the baseball and slinky race:
- The slinky falls faster than the baseball.
- There is no visible separation of the slinky's coils; it is fully contracted when dropped and remains fully contracted throughout its fall.
Possible explanations for metal slinky motion:
The following concepts apply both to the metal and plastic slinkies. The reason for this is because the geometry and physics of the two slinkies are similar; the definition of center of mass applies to both slinkies, and for both slinkies, coil motion is the primary method of transferring energy. Other methods identified, but ignored in this experiment, are friction within the slinky and friction due to air resistance. These concepts are important in determining explanations for why the slinky falls as it does.
- The slinky's center of mass falls at an acceleration very close to g. The slinky can be represented by a single particle with a given mass, position, velocity, and acceleration; this is the center of mass. It can be a point on the slinky or a location in space. The center of mass accelerates very close to g, since any particle released freely on Earth falls at an acceleration of g - Fdrag.
- When only one coil is held to drop the slinky, the center of mass (through symmetry) lies between the two end coils. From the center of mass reference frame, both the top and bottom move toward it at equal rates as the slinky collapses. We see this motion as the top falling toward the center of mass at an acceleration greater than g, and the bottom remaining still.
- As the slinky falls, it gains kinetic energy that is transferred through its coils. The method of energy transfer is a wave, but no wave can be seen in any of the plastic slinky vertical relaxed drops. This information was gained from the website
http://www.teachingtools.com/slinky/activ1.html, where two slinkies were made to walk down steps:
"After the race, ask why the smaller Slinky won. (As the Slinky moves down the steps, energy is transferred along its length in a longitudinal or compressional wave which resembles a sound wave that travels through a substance by transferring a pulse of energy to the next molecule. How quickly the wave moves through the Slinky depends on the tension and mass of the coil. The smaller the mass, the tighter the tension; the tighter the tension, the faster the wave speed. So, the wave moves faster through the smaller Slinky.)"
