a person in a lift falling freely under gravity feels weightlessness because the force of reaction between the person and the plane with which he is in contact vanishes. why force of reaction vanishes? will the person not be in contact with the floor of the lift?

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    It's not the contact touch between 2 surfaces that matters, but whether the body is going along a geodesic or not. Also see the equivalence principle – Avantgarde 2 days ago
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    Do a free body diagram on the person in the lift when they are both in free fall and when they are not in free fall (lift suspended by a cable. That should give you your answer. – Bob D 2 days ago

In order to understand why there is no reaction force on a person in the case of a lift in free fall versus a person in the case of a lift not in free fall (e.g., lift suspended by a cable), you should do a free body diagram on the person for each case assuming the person is in contact with the floor of the lift, and determine the lift reaction force for each case.

The key point you need to realize is the person has acceleration $g$ in the first case and no acceleration in second case.

Hope this helps.

You are imagining two surfaces touching and being attracted, like two magnest that make a force on each other. But two surfaces can be in contact just by chance and do not make a force on each other. That is what would happen in an elevator in free falling. Both, the elevator and you are falling with the same acceleration, that of gravity, and thus neither is pushing each other. In a general relativity context the description changes, and it is not the force of gravity that is acting on both objects, but that both objects are moving at constant speed along a geodesic of deformed spacetime, deformation that is originated by the mass of the earth.

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