# Fluid pressure on an object in open system

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1. Oct 14, 2014

### Toro

Hello everyone,

I have a physics question on pressure exerted by a fluid onto an object (see attachment). I would like to know how I can calculate the total pressure exerted on an object in a vessel by flowing fluid (water) with an open outflow. It has been too long since I took college physics, a class that I did not do particularly well in. I would be grateful to anyone who could point me in the right direction.

Tangential question: I'm also a bit confused about atmospheric pressure and the pressure in the human body. If the atmospheric pressure is ~700 mmHg, how can blood vessels (15 mmHg) have a lower pressure? I thought it was the other way around?

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• ###### pressure on object.pdf
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2. Oct 14, 2014

### SteamKing

Staff Emeritus
760 mm Hg is an absolute pressure reading for normal atmospheric pressure. The reading of your blood pressure taken by a doctor is a gauge pressure reading, where 0 gauge is the local ambient pressure. After all, if your body operated at a lower pressure internally from ambient conditions, every time you got a cut or scrape, you would blow up like the Stay-Puft Marshmallow Man.

3. Oct 14, 2014

### Staff: Mentor

Since the flow rate is so low, and the object is so close to the outlet, the pressure on the object is basically going to be atmospheric pressure (in this particular example).
Chet

4. Oct 15, 2014

### Toro

Thanks for the info Chet. If it wouldn't be too much trouble, what formula would you use to calculate pressure in my case? I was thinking of using the static fluid pressure equation, but I have liquid flowing from the top and exiting from the side. Surely there must be a separate equation to account for this.

5. Oct 15, 2014

### Staff: Mentor

It looks like what you are dealing with here is flow through a tubing of small diameter. In the system you described, there is a balance between pressure forces, gravitational forces, and viscous drag forces. The viscous drag supports the weight of the fluid as well as the pressure difference from beginning to end of the tubing. The pressure will vary linearly with distance along the tubing, from the exit of the reservoir above, to the bottom of tubing (where the pressure will be atmospheric). There is a static pressure variation in the reservoir above, so the pressure at the entrance to the tubing is the static pressure head in the reservoir. You can find the solution to this viscous flow problem in Transport Phenomena by Bird, Stewart, and Lightfoot.

chet