2 explanations for normal force

mishima

I have heard 2 informal explanations for the normal force during a situation like a book resting on a table:

1. The springiness of the atomic lattice in the table attempts to restore its original structure, thus pushing the book.

2. Since there are electrons on the outside of both the book and the table, the book is prevented from falling through (- repels -). Some even say this is the Pauli exclusion principle, not just electrostatic forces. Those particles cant occupy space already occupied.

Which one is correct, or are they both correct?

Then, if 2 is correct, say you wanted to float the book above the table using electrostatic forces. Would you need to put a charge on both the book and the table? Or could you just put a charge on the table and let induction push the book up (since all the electrons rush to the top of the book)? Thanks.

Drakkith

The springiness of the table is irrelevant. Even if it were a perfectly rigid object you would still get a repelling force between the atoms in the table and the atoms in the book thanks to their negatively charged electron clouds.

If you wanted to float the book a noticeable amount of distance above the table using electrostatics your would need to charge both the book and the table.

mishima

Ok, thanks. I had learned 2 as you said, but had seen 1 in some texts.

Drakkith

Well, it's not WRONG, it just doesn't explain the repulsive force at all. It itself is a result of repulsive forces within the object. (And maybe attractive too)