The bottom doesn't know it's falling.

This is the real slinky-drop demo, familiar from countless slow-motion videos and analyzed in detail by physicist Rhett Allain: release only the top of a stretched slinky and the bottom hangs in the air, unmoving, until a compression wave — carrying the "news" that the tension holding it up has changed — travels down and reaches it. Before that news arrives, the bottom's own weight is still being held up by exactly the tension it always had, so nothing about its situation has changed yet. The slinky's overall centre of mass falls at g the whole time regardless, exactly as Newton's second law demands for the system as a whole — it's only individual coils, not the average, that wait their turn. Wave speed here is v = L√(k/m) (equivalently √(kL²/m)): k in N/m, L in m and m in kg gives (N/m·m²)/kg = (kg/s²·m²)/kg = m²/s², so the square root is honestly in m/s. The travel time is then L/v, which simplifies to √(m/k) — note that L cancels out entirely, so stretching the slinky further doesn't change how long the bottom waits, only how dramatic the pause looks (the wave has farther to travel, but it also moves proportionally faster). k, L and m above are representative values for a typical metal slinky, not one specific product's spec sheet. Each coil in the animation is modelled as staying perfectly still until the wave reaches it, then beginning free fall from rest at that instant — a simplification of the true elastic dynamics, but one that reproduces the timing and the stillness this page is about.