Can someone please explain why the bottom of the slinky doesn’t fall to the floor until the top is finished falling down?
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The slinky is a spring. The natural state is for it to be compressed in a tight coil, so it wants to return to that state.
When you hold the slinky up from the top, the slinky extends due to the weight of the bottom. It extends until the compressive force of the spring is enough to hold the bottom up; the force that a spring can exert is roughly tied to how far it has been stretched/compressed compared to its natural/rest state.
So at the moment before you drop the top, the slinky is completely stationary. The spring is exerting enough force to hold the bottom from falling further, with that force being exerted on the top. You feel that force as the weight of the slinky pulling your hand down, which you (easily) resist to hold it still.
When you drop the slinky, there’s nothing holding the top up anymore. But the bottom is still being held by the stretch of the slinky. That force is enough to hold the bottom roughly in place against weight/gravity (if you watch carefully, it’s not absolutely stationary while the top is falling) until the slinky closes up and the momentum of the top of the slinky coming down pushes the balance of forces all the way towards weight/gravity downwards.
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