Observations made in the horizontal slinky drops:
- When stretched beyond horizontal relaxed length, the slinky contracts upon release.
- When contracted and allowed to drop horizontally, the slinky relaxes upon release.
- The slinky rarely changed its orientation from horizontal to vertical in these drops, as it often did in the vertical drops.
The most interesting motion in these drops did not occur during the drop, but after the slinky had collided with the ground.
- When the slinky landed on its corner (the bottom end of the slinky impacted the ground at an angle), it rebounded into the air.
- If it landed on its corner just right, one end of the slinky would remain on the ground as the other shot upwards.
- In some cases, the entire slinky bounced upwards after landing on its corner.
- When the slinky landed on its side, it would bounce horizontally (expand left and right) instead of bouncing upwards.
Observations made in the vertical slinky drops:
- In the vertical contracted drops, the slinky's motion appears to be very sensitive to motion at its bottom end. In all three clips, the slinky eventually rotates from a vertical to a horizontal orientation. The slinky completes this rotation much more slowly when the flat surface remains in contact with the bottom of the slinky for a shorter amount of time.
- In the vertical relaxed drops, the bottom end of the slinky has been shown to remain still until the top end collapses down on it.
- It appears that the top end flips over the bottom end in the vertical relaxed drops. The top end does not appear to start the bottom end moving, as in a Newton's Cradle-like scenario.
Possible explanations for plastic slinky motion:
Horizontal:
- When stretched beyond horizontal relaxed length, the spring restoring force causes the slinky to contract upon release.
- When contracted and allowed to drop horizontally, the spring restoring force causes the slinky to relax upon release.
- If the slinky lands on a corner, it experiences compression. The easiest way for the slinky to decompress itself is probably not to push against the floor, but to rebound upwards. This would require that the net force acting on the slinky from the floor is greater than both the force of gravity and the restoring force on the slinky.
- No explanation has been found yet for why the slinky expands horizontally when it impacts the ground.
- The slinky did not change its orientation from horizontal to vertical as easily as during the vertical drops, perhaps because air resistance is equally distributed on each coil of the slinky. For a vertical slinky, the greatest air resistance acts on the bottom end of the slinky, causing it to tilt over.
Vertical:
- In the vertical contracted drops, friction between the flat surface and the bottom end of the slinky causes the slinky to rotate in the horizontal plane upon release.
- The slinky rotated less horizontally when the flat surface was removed in the vertical contracted, nonsupported drops. This was likely due to the removal of the frictional force due to the flat surface. However, the rotation could not be eliminated because the slinky could not be dropped in a perfectly vertical orientation.
- In the vertical relaxed drops, the bottom end of the slinky remains still while the top part collapses down on it. This is the most complex and interesting motion observed yet in the plastic slinky. Some key points to understanding why this happens are:
- 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. The center of mass accelerates very close to g since any particle released freely on Earth falls at an acceleration of g minus the drag force.
- Because only one coil was 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 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.)"
