Free Body Diagram Q&A: Atmospheric Pressure?

[lin_howt]

Hi! Was wondering if anyone could enlighten me as to whether the atmospheric pressure is to be considered when drawing the free body diagram of a book resting on a table? In addition to the normal force and the weight, does atmospheric pressure need to be included?

 

[jcsolis]

mmm, I don´t think so...if you look on textbooks they only include the normal and the weight

 

[AlephZero]

There is no "right answer" to this type of question. It depends what you want to use your free body diagram for.

For a book on a table, the difference in air pressure between to top and bottom surfaces is small compared with the other forces, so you can usually ignore it.

On the other hand if you were weighing the book on a very accurate balance, you probably would want to include the resultant of the forces from the air on all the surfaces of the book (the buoyancy force) as a correction factor.

As another example, consider a free body diagram of a balloon resting on the table. The relative size of the weight of the balloon compared to the air pressure forces has a large effect on what happens.

 

[lin_howt]

Most textbooks do not include the atmospheric pressure without stating why. Anyway, given the size of the atomospheric pressure, even a small surface area would translate to a sizeable amount of force acting down on the book. If my objective is to include all the forces acting on the book, would including the atmopheric pressure give a more accurate picture?

 

[rootX]

Most textbooks do not include the atmospheric pressure without stating why. Anyway, given the size of the atomospheric pressure, even a small surface area would translate to a sizeable amount of force acting down on the book.

Consider two cases:
Case 1: An empty system
Case 2: An object is introduced to that empty system.

All other forces like N,W, and f comes into existence only in case 2, while disappear in case 1.
But the force due to atmospheric pressure is present in both cases.

So, I guess it has very little effect on the system, and hence you don't need to include it in FBD.

Secondly, it's impossible to get perfect results. And as FBD are much more used for practical purposes, so approximation methods(that can make things simpler) are preferred. like sin t = t

 

[bdrosd]

I wonder if lin howt is concerned about the air pressure acting on the sides and top of the book but apparently not on the bottom surface? Of course this is not the case, air can almost always get inbetween the book's bottom surface and the table and thus push upwards with almost exactly the same magnitude as the downward pressure force on the top of the book. If the pressure was only acting on the top and side surfaces, it would definitely need to be included in the FBD.