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A slinky (long spring of low spring constant) is suspended vertically by holding one of its end. When released, the slinky gets compressed in a peculiar way. The video (in slow motion) shows the upper end coming down fast whereas the lower end almost remains stationary. After initial compression, the slinky fall down in the compressed form.
Question: The slinky shrinks during fall because
Solution: Let us see the forces acting on the upper coil (U) and the lowe coil (L) of the slinky. The forces on the upper coil when it is suspended from support (hand) are
Similarly, the forces on the lower coil L are
Note that $a_U>a_L$. Thus, the upper coil moves down fast in comparison to the lower coil and the slinky gets compressed. The magnitude of the spring force $F$ decreases continuously from its initial value to almost zero (due to spring compression).
After initial compression, the spring force becomes zero ($a_U=a_L=g$). The slinky falls freely in the compressed form with an acceleration $g$.