Homework Help: Physics (Normal Force)

Posted by Ben on Thursday, October 1, 2009 at 2:01am.

When a book weighing 10 N is placed on a table, the table exerts a normal force of 10 N on the book. When a crate weighing 250 N is placed on the same table, the table exerts a normal force of 250 N on the crate. Explain the mechanism that allows the table to exert just the right normal force to balance the weight of the object. Is there any limit to how large the normal force can be?

This is what I have so far:

I am not sure about the mechanism, but for the second part, I said that there is a limit. This limit is until the table breaks due to the force. For the first part, I am not sure how to approach it. I have searched in my textbook but I've had no luck.

Thank you for your help.

Physics (Normal Force) - Count Iblis, Thursday, October 1, 2009 at 9:44am
I agree with your answer to the second part.

You can consider the first part of the problem from different perspectives. From the macroscopic perspective where you look at the book and the table on the scale where you don't see the individual atoms, you can describe the book and the table as elastic objects.

When you put the book on the midle table, the table deforms. Tensions build up in the table and on the boundary of the table and the book, the table exerts an upward force on the book. Very soon ater you put the book on the table, equilibrium is reached.

From the microscopic perspective, you can consider the table and the book as a collection of atoms. The forces the table exerts on the book are caused by the repulsion of the electrons in the book by those on the table. The reason that this happens is not simpy because the same charges repel, but because electrons cannot be in the same state.

When the book is pushing down on the table, the electrons are moving to higher energy levels so that they won't end up in the same states. This increase in energy yields the force that the table exerts on the book.

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