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Ok let us take an example of a book placed on a table and in first case let us say that it is on an aeroplane at rest. Since the book is at rest this means that the weight is balanced by the normal of the table.

But if the same ( book + table ) system is kept on a flying aeroplane (in horizontal direction with constant speed )at a great height ( comparable to earth's radius), then the weight of the body will be decreased but it is still at rest. This again means that the weight is balanced by the normal of the table. This means that the normal force has decreased in order to balance the weight.

But since normal force is the electric force , so why and how does the electric force decreased without altering the distance between the charges in the book and the table ??

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When the book rests on the table it compresses the table by an amount you would have difficulty measuring but nether the less the table (and the book) are compressed.
This means that the separation of the molecules with the table (and the book) has decreased from their normal static equilibrium separation so now each molecule is exerting a force on its nearest neighbour to counter the force exerted on the molecules which make up the table by the molecules which make up the book (weight of the book).

Think of a vertical spring resting on the table.
Now add a book on top of the spring.
The spring will compress until it exerts a force is exerted by the spring on the book which is equal to the weight of the book.
Now think of the molecules in the table all being connected together with little springs (bonds) and when the book is placed on the table the springs (bonds) are compressed.

As the aeroplane flies higher the compression of the table is less because the weight of the book is less.

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  • $\begingroup$ Can we use spring analogy for the normal force ?? $\endgroup$ – user262060 May 23 '20 at 11:04
  • $\begingroup$ @Ankit Kumar I do think that spring analogy works well. Normal force is basically preventing the book to just simply go through the table. So, suppose you have a spring, if it is acted upon by a smaller force, it compresses less and hence exactly balances the force by developing a spring force. As the external force increases so does the spring force. In case of normal force it's just that compression is on a very small scale, and it may be that there are multiple coupled spring acting together to oppose the book. $\endgroup$ – Abhinav Dhawan May 23 '20 at 11:56

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