Slinky

The initial potential energy at the top of the stairs makes the spring descend the stairs.

However, the law of conservation of energy itself does not explain the specific manner with which the spring moves. It is the law of conservation of momentum that also applies. When it descends step by step, the spring has also some component of horizontal momentum. It is this component of horizontal momentum which allows for overcoming the dead-point; the end of the spring movement is mainly perpendicular, but to some extent it is also horizontal.

Analysing this from the point of view different forms of energy, after the descending one step, the spring accumulates elastic energy, but not in elongation of the spring but squeezing it. The squeezing effect is harder to notice, since the deformation is minimal but Young's modulus at such circumstances is great. It is not the deformation of the spring as such, but of the material it is made of.

Finally, the motion of the spring follows longitudinal wave of a specific nature; it is not the wave that moves down the length of the spring, it is the spring that moves down the length of the wave, which is very small.

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