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Imagine I pour some milk inside the elevator and then I cut the cable to allow the elevator to free fall, does it immediately float or only when the elevator speed has reached 9.81m/s/s? Oh and assume the floor is coated with Teflon so the milk cannot stick.

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To clarify, the speed of the elevator is not important, it is the acceleration of the elevator which is $9.81\,\mathrm{ms^{-2}}$.

Now, your question is a great excuse to talk about the equivalence principle; one of Einstein's thought experiments that helped him create general relativity. The equivalence principle states that if you are in free fall in a small lab over a short time, like in a falling elevator, in a gravitational field then physics to you is indistinguishable to physics for someone else, also in a small lab for a short period of time, in complete vacuum (far away from any gravitational fields).

Why is this? Pretend you are falling in the elevator. In your hand is an apple. You extend your hand and "drop" the apple. The apple is falling at the same rate as you are. So you won't see the apple "fall". Relative to you it appears to "float", precisely as if there were no gravitational field at all. This is the equivalence principle.

What does this imply for your question? Well the milk is falling at the same rate as you are. However, Newton's first law tells us that objects at rest remain at rest. This is still true in a falling elevator (in fact it is especially true since a falling elevator is an example of a true inertial frame). So the milk will remain in a puddle on the floor of the elevator, unless you splash it or something. If you do it will not appear to fall back towards the floor of the elevator, as it is already falling at the same rate as you.

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You can experiment with a container filled with water dropped from a height. It will not be something like floating in air midway between the height of elevator, theoretically, it will just lose contact with the floor.

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