# Does centrifugal force caused the tides?

1. Mar 10, 2013

### hafind

First time poster, thanks for help

I wonder if low tide is caused by centrifugal force at noon time and high tide at midnight?

The centrifugal force pushes sea water toward the center of the Earth at day time, away from it at night time. So, a man weights more at day time than night time.

Am I have a point?

2. Mar 10, 2013

### davenn

hi there
welcome to PF,
the gravity interactions of the Earth, Moon and Sun cause the tides
You may not realise that the high and low tides are NOT at the same time each day ?

Have a read of this wiki page on tides

cheers
Dave

3. Mar 10, 2013

### A.T.

You can blame the centrifugal force for the equatorial bulge in the rotating restframe of the Earth. But that is not the tides.

Some also say centrifugal force causes the moon-opposite tidal-bulge in the rest frame of the Earth's center, but that is missleading. The gravity gradient of the Moon's gravity explains all the deformation, so there is no need to invent additional causes in terms of inertial forces.

4. Mar 10, 2013

### molly cruz

tide and gravity

Because I was just pondering tides I must add the possibility that occurred to me yesterday; living as i do by the sea: If the Earth is a flexible ball, suppose the shape changes with the rotation, in response to the moon and sun, and thus forces the water to respond?

5. Mar 10, 2013

### Staff: Mentor

Welcome to PF!
Correct!

http://en.wikipedia.org/wiki/Earth_tide

6. Mar 11, 2013

### hafind

Thanks. I still don't understand.

The centrifugal force pushes everything on Earth away from the Sun. So the water facing the Sun will become low tide, the other side high tide. Moon gravity and Earth movement play part of the game.

Do you think centrifugal force has nothing to do with tide? Maybe we can put a gel ball on a turn table and see if the ball changes shape?

7. Mar 11, 2013

### A.T.

Between high and low tide there is 90° not 180°.
http://en.wikipedia.org/wiki/Tide

8. Mar 11, 2013

### Staff: Mentor

In addition to the fact that there care two high tides, not one, tidal forces are observed with objects in free fall, where there is no centrifugal force.

9. Mar 11, 2013

### ZapperZ

Staff Emeritus
Y'know, you can easily check if your prediction is correct. There are high tide and low tide timetables for many parts of the world. I can assure you that these do not match your time period.

Zz.

10. Mar 11, 2013

### hafind

If I have the tools I will try to use a line to hang a steel ball, measure the tension of the line at mid day and mid night. My prediction is the ball weights more at day, by 2 x its mass x 0.0006/9.08.

???

11. Mar 11, 2013

### D H

Staff Emeritus
The too long, didn't read answer: The tides are not caused by centrifugal forces.

You are using the term "centrifugal force" without apparently understanding it.

So let's back up a bit. First thing to note: The centrifugal force is an inertial force (or pseudo force or fictitious force).

The preferred frames of reference in Newtonian mechanics are the frames in which all three of Newton's laws of motion hold, the inertial frames of reference. There is no such thing as the centrifugal force in an inertial frame; all of the fictitious forces vanish in an inertial frame.

This means you can't attribute cause and effect to a fictitious force. What one can do is use fictitious forces as a convenient way of explaining some behavior. For example, it's far easier to explain why hurricanes rotate from the perspective of a frame rotating with the Earth than it is from the perspective of an inertial frame.

Back to the tides, the explanation from the perspective of an inertial frame is quite simple. Imagine two drops of water, one at the point on the surface of the Earth closest to the Moon, the other at the point on the surface of the Earth furthest from the Moon. Gravity makes both drops, along with the Earth as a whole, accelerate toward the Moon.

Gravitation is an inverse square law. The acceleration of the drop closest to the Moon is greater than is that of the Earth as a whole. That drop closest to the Moon is thus pulled away from center of the Earth. The acceleration of the Earth as a whole toward the Moon is greater than is that of the drop furthest from the Moon. The center of the Earth is thus pulled away from that drop furthest from the Moon. Another way to look at this: The drop furthest from the Moon also is pulled away from center of the Earth.

It's oftentimes convenient to explain the tides from the perspective of a frame of reference with it's origin at the center of the Earth. The Earth as a whole is accelerating toward the Moon (and the Sun, and also toward every other mass in the universe). This means an Earth-centered frame is not an inertial frame. It's an accelerating frame, and hence some kind of fictitious force is needed to explain behaviors in this frame. This is the accelerating frame fictitious force. From the perspective of this (accelerating) Earth-centered frame, the tidal forces look like this:

This fictitious force is *not* the centrifugal force. The accelerating frame fictitious force is a uniform force, one that is constant in direction and magnitude regardless of location. In comparison, the centrifugal force results from a rotating rather than accelerating frame. The centrifugal force is proportional to distance from the center of rotation and is directed away from the center of rotation.

Some oceanographers, and even some oceanography textbooks, try to explain the tides using the concept of rotating frames and the centrifugal force. This is, to be blunt, an erroneous explanation.

12. Mar 11, 2013

### ZapperZ

Staff Emeritus
And then you have to reconcile to the FACT that this doesn't match the tidal pattern. Now what? Do you still hold on stubbornly to your idea despite the physical evidence? Or do you now examine that there are OTHER factors that dominate the tidal pattern than what you think?

What do you think a scientist would do?

Zz.

13. Mar 11, 2013

### Staff: Mentor

Hafind, the tides are sync'd with the moon's position, not the sun's (mostly). In that case, the only possible centrifugal force effect would be the earth's wobble around the earth-moon barycenter. So you'll have to time your experiment based on the position of the moon.

But anyway, you won't find any deviation, nor does this line of thought help explain the existence of two high tides.

14. Mar 11, 2013

### hafind

Really appreciate everyone's help!!! I understand it now, maybe 90%. Tooo much to learn, I only went to college 1 year.

Thanks again, please have a great day!

15. Mar 11, 2013

### olivermsun

It may be worth mentioning that the timing and amplitude of the (real) tides are not just the result of the positions of the sun and moon, but also have quite a lot to do with the shapes of the ocean basins. Of course, if you view the surface tide as a propagating shallow water wave rather than an ideal pair of bulges positioned around the earth, then I suppose it would not be incorrect to say that centrifugal force has some effect on the tide.

Out of curiosity, are there really oceanographers who argue that the tides are "caused" by something other than the sun and the moon, or is this a sort of urban legend among physicists?

Last edited: Mar 11, 2013
16. Mar 11, 2013

### A.T.

17. Mar 11, 2013

### olivermsun

:rofl:
Does anyone know which textbooks the confusing pictures are actually taken from?

Edit: I see from some further web searching that kind of incorrect explanation existed at the Texas A&M Oceanography website (oceanworld.tamu.edu) circa 2005, but I cannot find them now, and the online Physical Oceanography textbook seems to have a correct explanation invoking the tidal potential. I'm still curious where these explanations originated, and why they were apparently a subject of considerable controversy in 2005.

I also like this line by D. E. Simanek from the first link above: "The folks who do tidal measurements don't get into the physics theory much."

I suppose if he means the people who literally read the numbers off the tide gauges, then maybe he's right, but otherwise... :rofl:

Last edited: Mar 11, 2013
18. Mar 11, 2013

### D H

Staff Emeritus
They do attribute the tides to the Sun and Moon, but they do so badly. Very badly. At least in my opinion. You be the judge. Here's just a tiny sampling of what you get if you go to books.google.com and search for "tides centrifugal force":

The U.S. government's National Oceanographic and Atmospheric Administration (NOAA) starts out right here, but get it wrong on the following page. NOAA not only gets it wrong again here and here, they display a fundamental misunderstanding of the concepts of orbits, too. Centrifugal force is not needed to explain the tides, and it is not needed to explain orbits. (Misunderstanding orbits is apparently widespread amongst oceanographers).

The problem is our teachers. Students don't know the correct answer to what causes the tides (gravity gradient) because most teachers don't know -- even subject matter experts. See Jouni Viiri, Students’ understanding of tides, Phys. Educ. 35(2) March 2000.

19. Mar 11, 2013

### olivermsun

In your links above, I noticed that the books were written by a geologist, a marine biologist, a geophysicist, and an expert in marine biomechanics. The NOAA website meanwhile references a textbook written by another geologist.

Also, the explanation in the geophysicist's book actually looks similar to that (briefly) given in a lecture by Feynman.

I know that all these people may all look like "oceanographers" to the layman, but I wouldn't necessarily want to extrapolate from this sample to infer a "widespread" misunderstanding of tides among (physical) oceanographers (who study the tides, among other things).

20. Mar 11, 2013

### D H

Staff Emeritus
First off, I challenge you to find an oceanography textbook that properly explain the tidal driving forces. There are a few that get it right, but there are far more that get the explanation fundamentally wrong.

Secondly, by a happy set of circumstances, sometimes three wrongs do make a right. Even though that centrifugal force explanation is wrong, wrong, wrong, the end result is correct.

Thirdly, that two bulge theory of the tides is itself fundamentally flawed. While that is how tides would behave on a water world planet, that is not how the tides behave on our planet, with only 70% of the surface covered by water. That 30% percent of the surface that is land has a profound effect on the tides. The two bulge theory of the tides (Newton's equilibrium theory) cannot even come close to explaining why, for example, that no matter what time of day it is, you can always find some spot in the North Sea at high tide. There aren't two tidal bulges in the oceans. There are instead a number of amphidromic systems.

What's needed to explain the tides on our planet is a dynamical theory of the tides. Laplace started developing this theory. George Darwin (Charles Darwin's son) and A.T. Doodson pretty much put the finishing touches on the theory. Modern oceanography science extends this work by better modeling and observations of those amphidromic systems.

The theory outlines how tides behave in terms of frequency domain analysis. Those driving frequencies have different magnitudes and different lags at different points on the surface of the Earth. The dynamic theory of the tides explains what those driving frequencies are and provides a big picture view of those amphidromic circulations. However, modeling the magnitudes and phase lags of those tidal forcing functions at any specific point on the surface is very ad hoc.