# Why does normal force work?

I've got a lot of question about normal force. First, what causes normal force. Next, how do the object know how much normal force to exert. Lastly, how is this not an application of Newton 3rd law. Imagine a book on a table. The book is exerting a downward force on the table(weight), the table is exerting a normal force on the book(upwards) . People keep saying that they act on the same objecr, the book, but the book's weight is acting on the table, so why is this not Newton 3rd law

The normal force is exactly what it needs to be in order for the body to obey some constraint in motion or configuration.

If left to its own devices a block would free fall and its height would decrease by some amount every second. But when it rests on a table, its height remains constant, or it would go through the table. The normal force needed is exactly that to make this condition happen.

For example, if the table was on an accelerating elevator then the prescribed motion of the block would require a normal force different from the weight of the block.

Or when a roller coaster does a loop, the car accelerates downwards, but more than it would have if it was in free fall. Thus a normal force exists to push it downwards more and thus ensuring contact with the rails (since during the loop they can only push downwards) and maintaining control of the car.

In cases such as this one, the normal force arises due to objects resisting being combined into the same volume. Regardless of what is the apparent cause, when two solids are in contact, they will resist occupying the same volume.

If some other force like gravity brings two solid objects together, they are stopped from occupying the same volume. Therefore, there must be another force involved. This force is defined as the normal force and as such it must point perpendicular to the surface of contact.

This force is not a valid 3rd law action-reaction pair. For a valid 3rd law action-reaction pair, you need two forces only, which have the same magnitude, but opposite directions. While the earth exerts a gravitational force on the book, the book does the same to the earth. That is,

$$F_{Earth \ on \ book} = – F_{book \ on \ Earth}$$

And

$$F_{contact \ force \ of \ table\ on\ book} = – F_{Contact \ force \ of\ book \ on \ table}$$

So there are more forces than we would need for a valid action-reaction pair. Think about if you removed the table. Will the book still experience a gravitational force? The answer is yes. Think about if we ignored gravity, and you tried to push the book into the table. Will they react to each other still? The answer is yes.

• That is not true, as fluids exhibit normal force in the form of pressure. Commented Jan 11, 2021 at 3:49
• Force and pressure are different things. "force in the form of pressure" is nonsense.
– nasu
Commented Jan 11, 2021 at 8:20
• @JohnAlexiou I am not an expert in phsics, but i do know that force an pressure are seperaate things.
– user276724
Commented Jan 12, 2021 at 1:50
• @mary_stein Yes. We know. The statement "fluids exhibit normal force in the form of pressure" is a non-sequitur. $P=F/A$ and you should not let that comment confuse you. Commented Jan 12, 2021 at 1:54