# Thread: Why does water stay level to gravity all the time?

1. I have being trying to figure out why does water surface lie perpendicular to gravity?

Even if I tilt my bottle of water, it follows this rule.
How does the water 'know' which way Gravity is?

2.

3. The gravity pulls anything to it's center of mass (highly simplified, do not take this to be true except for the purpose of this question). As such, it attracts the molecules of water in you bottle too. If you tilt your bottle, the water would still be pulled towards the center of the gravity, rather than to your bottle bottom.

4. Hmm, thats interesting...

But say we drop the bottle down an inclined surface, I believe the water will be parallel to the inclined surface.

Gravity still is downwards why is it that the level changes?
Did the direction of the acting gravity change?

5. Do you mean dropping the bottle, or emptying the bottle on that surface. If you would drop the bottle, than the water would still be in the direction of the surface (except, of course, for disturbances cause by movement of the bottle).

6. Erm yes I meant drop.

So say for bottle on the inclined surface before dropping would be level with the floor.

But once we let go it would be level with the inclined surface right?

How do we explain this phenomenal?

7. No, it wouldn't be leveled with the inclined surface. I find this hard to explain without a sketch. You see, Newton's first law states that every object seeks to remain in the state of moving it had. As such, when you drop the bottle, and it starts to roll down that same inclined surface with the acceleration of , the water inside the bottle tries to retain it's state of motion, meaning that it tries not to move. And since the force on the water has a in it's equation, the water will be leveled with the inclined surface. But only for the direction of the bottle's moving. When the bottle stops moving, even if on the inclined surface (hitting an obstacle, for example), the water will level with the surface again.

8. If I understand your question, it's just because it takes time for the water to level out, and as it rolls down hill, it's getting pulled up the sides of the bottle. It wouldn't be level with the hill, it'd be somewhere between that and down (and not completely level either).

Erm why would the water get pulled up the side of the bottle?
Is it because the bottle acts a force on the water?

You said it wont be completely parallel with the inclined surface. Is that when you consider friction?
I was thinking if one neglects friction it will be parallel, why would it not be?

Sorry if I am asking a lot of question.

10. There are no stupid questions, and you need not to apologize. However, you really should learn about the Newton's second law. Friction would be the case here, but almost irrelevant.
MagiMaster, you mean the capillarity? Well it would be included, but I do think that the inertia would have far more influence, especially if the bottle is somewhat larger.

11. I suspect if the equal amount of gravitational force to the earth was generated by something the size of a pea that the water would attract to it to conform to the shape of the pea as opposed to a flat plane. So in a sense the water stays level (flat) because the source of gravity (the earth) is relatively flat. naturally if your bottle was several miles long the water would level off in a slight arc.

Just a guess of course.

12. Originally Posted by tikai
I have being trying to figure out why does water surface lie perpendicular to gravity?

Even if I tilt my bottle of water, it follows this rule.
How does the water 'know' which way Gravity is?
Because water,beibg a liquid cannot sustain a shear stress.

This is not a perfect relationship and surface tension effects do creaye a meniscus near the container walls.

13. Originally Posted by Sindrato
There are no stupid questions, and you need not to apologize. However, you really should learn about the Newton's second law. Friction would be the case here, but almost irrelevant.
MagiMaster, you mean the capillarity? Well it would be included, but I do think that the inertia would have far more influence, especially if the bottle is somewhat larger.
I'm imagining a bottle rolling sideways down a hill. I'm not sure what the full picture of all the forces involved is, but I know the water would be tilted slightly uphill due to a combination of friction, surface tension and inertia. I don't know if this tilt is significant though.

14. The tilt of the water in the bottle is only due to the momentum of the bottle rolling downhill at speed. If the bottle where to roll at 1kph the water would not do what you are envisioning.

15. Basically, for the water to be tilted, you must have acceleration. If the friction would cause no acceleration, than the water wouldn't be tilted.

16. Originally Posted by tikai

Erm why would the water get pulled up the side of the bottle?
Is it because the bottle acts a force on the water?

You said it wont be completely parallel with the inclined surface. Is that when you consider friction?
I was thinking if one neglects friction it will be parallel, why would it not be?

Sorry if I am asking a lot of question.
When the bottle is stationary, you could say that the water is pushing down on the bottle. (Or you could say the bottle is pushing up on the water. That's an equally valid observation.)

If the bottle is in free fall, then the water is no longer pushing down on the bottle (and the bottle is no longer pushing up on the water) , because gravity causes both the bottle and the water inside to fall at the same pace. So, the water inside is free to do whatever it wants. Relative to the bottle, it is as though it were weightless.

If the bottle is sliding down an incline, then that's kind of a partial free-fall situation. The surface of the incline is pushing on the bottle from an angular direction, so the bottle pushes on the water from an angular direction, instead of pushing straight upwards.

17. Originally Posted by (In)Sanity
I suspect if the equal amount of gravitational force to the earth was generated by something the size of a pea that the water would attract to it to conform to the shape of the pea as opposed to a flat plane. So in a sense the water stays level (flat) because the source of gravity (the earth) is relatively flat. naturally if your bottle was several miles long the water would level off in a slight arc.

Just a guess of course.
That effect depends only on how far you are from the center of the pea and how long the bottle is, not on the size of the pea. External to a uniform sphere the gravitational field is the same as that of an equal point mass at the center of the sphere.

A large bottle willhow some curvature, which is why the horizon over the ocean is curved.

18. Originally Posted by DrRocket
Originally Posted by (In)Sanity
I suspect if the equal amount of gravitational force to the earth was generated by something the size of a pea that the water would attract to it to conform to the shape of the pea as opposed to a flat plane. So in a sense the water stays level (flat) because the source of gravity (the earth) is relatively flat. naturally if your bottle was several miles long the water would level off in a slight arc.

Just a guess of course.
That effect depends only on how far you are from the center of the pea and how long the bottle is, not on the size of the pea. External to a uniform sphere the gravitational field is the same as that of an equal point mass at the center of the sphere.
Ahhh yes, but of course. I failed to state how far away the bottle was from the source of gravity (the pea). All perfectly logical.

19. Thanks everyone for the reply, I think I get it now.

20. But isn't tikai correct that the water level will be parallel to the inclined surface (disregarding friction and air resistance) while accelerating down the surface? And if the bottle (after reaching the bottom) is allowed to de-accelerate up a second inclined surface, wouldn't the water also be parallel to the second inclined surface while de-accelerating? And wouldn't this be true regardless of the angle of the inclined surface?

21. Why is this thread in "New hypotheses and Ideas"? Should it be moved, or am I missing something?

22. Water isn,t level to gravity.
Go stand in a bus that stands still take a leveler and keep it horizontal.
Forpeople outside the but the leveleris also horizontal.
Now the bus accelerates to keep the leveler horizontal according to what it shows you have to rotate it to an angle compared with horizontal outside the bus.

If it is kept in same position as it had (for people outside the bus) related to earths horizontality. The water and air will balance different inside the leveler.
What is horizontal in the bus is a resultant from gravity and acceleration by the engine of the bus.
Horizontality on earth is also a resultantout of different direktions ; earth, moon, sun....all together bring the tidal movements. and the resultant hasn,t a fixed direktion or strength. The water reacts as to how it changes similar as in the bus.

But there is a delay in this. The bus can go from zero accelleration to say 1 m/s^2.

But the water and the air can,t change their balance also instantly. There is a certain delay or....inertia as it is often referred to. This means it is not always horizontal or better to say it never is. Everything strives for dynamic balance but never reaches a static balance so to speak.

If the water at sea (and with that any water even in a bottle) would be leveled it would imply static balance and also direkt influence from a distance. That would mean so called "speed of gravity" as an influence works immediate instead of according to "speed of light". That would mean no inertia and therefor it,s not realistic.

23. Not entirely related, but this video came to mind:

24. There are no stupid questions, only inquisitive idiots.

25. Originally Posted by Osby
Not entirely related, but this video came to mind:

That is awesome.

It also illustrates well what I was saying. The orientation of the water is determined by the force of the glass pushing on it from below, not necessarily by gravity. The "centripetal force" of the airplane's roll maneuver means the plane is pushing on the glass, which is pushing on the water.

In gravity situations when you're not in free fall, the force of gravity is always opposed by a "N" force of something pushing against it. So normally, the N force of the ground pushing against the glass, which pushes against the water is what gives it its shape.

26. Originally Posted by Osby
Not entirely related, but this video came to mind: