# Simple Fluid mechanics question, Need Help

1. Aug 27, 2013

### Bassel

I've been solving fluid mechanics problems and i encountered two problems that made no sense to me. Both problems that ask about the force done by water on a VERTICAL wall under the level of water. One problem was solved by taking the integral of the gauge pressure and multiplying it by the area while the other was solved by also taking the integral of the gauge pressure and multiplying it by the area of the wall but after finding this value, the force due to atmospheric pressure was added. What happened There?? why two similar cases are solved in different meanings, anyone got any idea?

2. Aug 27, 2013

### abrewmaster

The only difference that I can see between these two scenarios is that one is using gage pressure and the other is using absolute pressure. Maybe the problem statements have a hint as to why they were solved that way? My only guess as to why this was the case was possibly because one problem statement specifically asked for the force done by the water where as the other one asked for the total force?

3. Aug 27, 2013

### Bassel

YESS EXACTLY after looking to the problems again i realized that one asked about the total force and the other asked about the force done by water. but the problem is i don't know the difference and that is why i didn't realize the difference. Can You please give me a summary about the differences please between asking about force done by water and total force ? thanks alot man sorry for bothering you.

4. Aug 27, 2013

### abrewmaster

Atmospheric pressure is the pressure caused by the atmosphere which is typically considered to be ~101325 Pa. Gage pressure is basically the difference between this number and the pressure that the point of interest is at since "normal conditions" are at atmospheric pressure. The force that is being applied only from the water is the force from the gage pressure; however, this value does not take into account the pressure that the object would typically experience if the water wasn't there and therefore does not take into account this force. If there was no water the wall would still see some force on it because of atmospheric pressure so if you want to find the total force you have to take that force into account as well. The main difference is one only asks for the change in force under normal circumstances where as the other one asks you for the total.