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If two books are placed on top of each other on a table, why would it be incorrect to say that the weight on the upper book acts on the lower book? I thought that the weight of the upper book would act on the lower book, but my professor said that the normal force instead of the weight of the upper book would act on the lower book. I didn't understand his explanation.

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  • $\begingroup$ Weight is acted by the earth. $\endgroup$ – lucas Apr 21 '16 at 8:48
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Both books are individually attracted by the whole mass of earth. That gives you the force $mg$. In principle also the books attract each other via gravity, but that force is so small that you can safely neglect it.

Let's start with the lower book. It sits on the table, gravity is pulling it downwards. The table then resists the compression by the book exerts a normal force onto the book. The gravitational pull goes downwards, the normal force goes upwards. Both forces cancel each other out exactly, the book is at rest.

Now add the upper book. The gravitational pull also be there for the second book. The lower book resists the compression by exerting a normal force onto the upper book. This keeps the upper book at rest. The lower book is now pressed harder onto the table. To resist the compression, the table has to double its normal force now.

To sum up: The upper book has the following forces:

  • Gravitational pull from the earth (down)
  • Normal force from lower book (up)

The lower book has the following forces:

  • Gravitational pull from the earth (down)
  • Pressure from upper book (down)
  • Normal force from table (up)
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