Although its movement may look simple, from a scientific point of view the motion is quite complex. 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 depends on the spring constant and the mass of the metal. Other factors, such as the length of the slinky, the diameter of the coils and the height of the step must be considered to completely understand why a slinky moves as it does. James originally developed the Slinky for the Navy as an anti-vibration device for ship instruments.
Sound waves are longitudinal waves.
Waves in the stretched strings of musical instruments, upon the surfaces of liquids, and the electromagnetic waves which make up radio waves and light are transverse. Friction always acts to slow movement and if no other force is applied, it will bring motion to a stop. Resources Ardley, Neil. Muscles to Machine. Glouchester Press, 1990.