# Fluid Mechanics, Pressure

1. Jun 15, 2007

### tsochiu

There is a u-tube which its diameter on the left hand side is smaller than the right hand side.
It is filled with liquid.
As the atmosphere pressure is equal in the two side, and force = pressure*area.
I want to ask, why the liquid in the u-tube is not pushed to the left side as the force from atmosphere on the right side is larger than left? Or what is wrong in my concept?

2. Jun 15, 2007

### Staff: Mentor

While it's true that the force of the atmosphere is greater on the right side surface, that's not the only force acting on the liquid: The sides of the tube also exert an upward force on the right hand side. (The details depend on the exact shape of the tube.) The net force on the liquid will be zero.

3. Jun 15, 2007

### tsochiu

Sorry, I cannot understand it clearly, could you give me an example on how the force is balanced? With a free body diagram will be better. Thank you very much

4. Jun 15, 2007

### Staff: Mentor

Why don't you provide a diagram of the U-tube in question.

Last edited by a moderator: May 2, 2017
5. Jun 15, 2007

### tsochiu

Your web page is really helpful. I can understand what is happening in the u-tube now.
That is when the u-tube start to narrow,the wall of the u-tube will give a reaction force toward the liquid which will cancel out the extra force due to the wider mouth.
Thanks for your help, it solve my great problem with pressure.

6. Jun 15, 2007

### Staff: Mentor

If you push on an object with twice the force with which you push on an object of half the mass, which will accelerate faster?

7. Jun 15, 2007

### Staff: Mentor

Exactly.

Regardless of the mass of the fluid, if there's a net force on it, it will accelerate. The problem here is in thinking that the only force on the fluid is that due to atmospheric pressure.

8. Jun 15, 2007

### Staff: Mentor

It was an analagous situation, not an analysis of this situation.

9. Jun 15, 2007

### Staff: Mentor

I guess I didn't see the analogy.