# How does water exerts pressure on any object that is immersed in it?

1. Jul 14, 2013

### threy

How does water exert pressure on any object that is immersed in it?

How does water exert pressure on any object that is immersed in it?

Last edited: Jul 14, 2013
2. Jul 14, 2013

### Staff: Mentor

How? The weight of the water above pushes on the object from all sides.

3. Jul 17, 2013

### lightarrow

Explained in a different way: due to fluid weight, the pressure inside a fluid is lower above and greater below (it increases from athmospheric pressure at the interface athmosphere/fluid to the maximum value at the bottom of the fluid container) so an object immersed in a fluid (in a gravitational field directed downside) receives from the fluid portions below a greater pressure than that from the fluid portions above.

4. Jul 17, 2013

### Staff: Mentor

It's also important to be aware that, at any given location in the fluid, the pressure is isotropic, and pushes equally in all directions.

5. Jul 17, 2013

### dipole

I don't think this is true. In water, for example, the force of gravity creates a pressure gradient in the water normal to the surface. This is where buoyant forces come from - the pressure is greater on the bottom surface of an object then at the top surface, and the difference in pressure creates a force which pushes it upwards. If this force is greater than the force due to gravity, it floats.

6. Jul 17, 2013

### Staff: Mentor

As a PhD engineer with 45 years of fluid mechanics experience, I can assure you that it is true. Otherwise, how do you think water exerts a horizontal force on a vertical dam?

7. Jul 17, 2013

### dipole

I misread what you wrote before - I thought you were claiming that the pressure exerted on some object is equal in all directions, because the OP was originally asking about an object of, presumably, finite size. I don't dispute you when talking about a mathematical point though.

8. Jul 17, 2013

### Buckleymanor

So is the horizontal force of the water the same at the bottom of the vertical dam as the top?

9. Jul 17, 2013

### Staff: Mentor

No. The pressure increases with depth below the surface, so the horizontal force (on a unit area) will be greater at the bottom of the dam.

10. Jul 17, 2013

### Buckleymanor

I am not sure what Chestermiller is saying.
Fluid flow is isotropic so does this apply to the pressure difference between the top and bottom of the dam also.
It does not look like it should.

11. Jul 17, 2013

### Staff: Mentor

At any given location, the pressure is the same in all directions. Different locations, such as the top and bottom of the dam, can certainly have different pressures.

12. Jul 17, 2013

### Staff: Mentor

Thanks Doc Al for clarifying what I was saying.

Chet

13. Jul 18, 2013

### Buckleymanor

How can it be there must be a gradient if it's different at the top location.
I still don't grasp it.

Last edited: Jul 18, 2013
14. Jul 18, 2013

### bahamagreen

Two kinds of "sameness" here:

Sameness in all directions (of pressure)
and
Sameness of value of pressure

"At any given location..." is a conditional phrase that means the chracteristic or property to be described "...the sameness in all directions of pressure" applies to each of the individual possible locations.

The "sameness in all directions" applies to each location, the pressure of that sameness is not necessarily the same in all locations.

Two locations can each have pressure push the same in all directions and those be two different pressures.

15. Jul 18, 2013

### jbriggs444

The existence of a non-zero gradient means that over a small distance there is a small variation in pressure. The limiting ratio of the pressure difference divided by the distance difference as the distance gets smaller and smaller is called the gradient.

[There are some other details involving the existence of a unique limiting ratio and about turning this into a vector-valued function that need not be discussed unless you are interested].

But if the distance is zero the pressure difference is zero.

16. Jul 18, 2013

### Buckleymanor

Probably what I do not understand is the distance being zero.
I can not see a zero distance unless there is no substance.
Is it a theoretical construct similar to infinity but in reverse.

17. Jul 19, 2013

### CWatters

Yes.

You could measure the pressure at two points of different depths. Then plot a graph of the pressure difference vs distance between the points. Extrapolate the graph towards zero separation and you will find the predicted pressure difference is also zero.

18. Jul 19, 2013

### Staff: Mentor

Buckleymanor: Would it be correct to say that you haven't had calculus yet, and thus do not understand the concept of a derivative?

Chet

19. Jul 20, 2013

### Buckleymanor

Thanks that makes some sense to my misunderstandings.

20. Jul 20, 2013

### Buckleymanor

Would that make a physical difference.