I'm trying to look for something that I'm not used to. My search is related to Tidal forces and the purpose is: making a qualitative explanation (@ High School level) of this phenomenon, using daily life situations to "build the model in my students' mind". One idea one could come across is: fishing schedules at different moon phases...

To do so, I explained this to my 80 years old grandmother and she made an excellent explanation from the questions I did to her. In summary:

  • Could you imagine you are in a space without gravity? Could you tell me what do you see when we pour water? What shape do you see? (Expected: a sphere. She invoked gravity)

  • What do you think about the sphere, in this experiment in which you are now, eyes closed no gravity, if now I add a mass near to the bubble? (Expected: a deformation. She said: I'd see a "tear"/"eggplant" shape, pointing to the mass object)

  • And what do you imagine it would happen if now you add a solid ball inside the deformed tear bubble you see? (Expected: a change in the shape towards the center. She said: what I expected! "the deformation would be less perceptible because the solid ball is pulling water to its center).

  • Why does it points to the external mass? (Expected and answered: gravity is pulling towards it. She said: it would be important to consider each mass to describe what would happen)

  • Now I asked what would happen if the water bubble was now made of another material: I mentioned oil. She said the deformation would be modified. Now I asked her to move the external mass, orbiting around the deformed bubble with the solid ball inside it. She said that the "bulge" (can't translate the exact word she used because we didn't talk in english...) would be moving, following the orbit of this object. She didn't mention a lag angle or time, and I find it OK. because she only studied physics @ school when she was in middle school...

  • I tried to ratify the position of the solid ball inside the deformed bubble, and she said again it'd be @ the center. I asked her if she can relate it to any phenomenon she knows and she explained me about tides on the sea, telling me that there are 4 each day: two high and two low tides. This was very surprising.

Having this in consideration, I'm convinced that 16 year old students can do great at this, so I'm doing a video. I thought that a good idea would be to bring a daily example of this "water bubble" concept, floating in space. The only resource I find is this kind of experiment:


but, obviously, it won't show what happen when a mass object is near to the bubble.

Question: Can you think about an example, analogy or metaphore to illustrate what is happening as it's understood? for instance: using ferromagnetic fluids to explain the effects of a force on a fluid?

  • 2
    $\begingroup$ Where did you get this video? Did it not come with some explanation? $\endgroup$
    – R.W. Bird
    May 2, 2022 at 20:51
  • $\begingroup$ @R.W.Bird yes indeed, it comes from ISS&NASA but its purpose is not giving an explanation about tides. $\endgroup$
    – nuwe
    May 3, 2022 at 17:30
  • 1
    $\begingroup$ Interesting. But it is not clear how the gravity pulling towards the external mass would result in two high (and two low) tides and not just one. I think this is the most difficult part of the story both for grandmothers and students. $\endgroup$ May 4, 2022 at 5:33

2 Answers 2


I can think of two examples, however Im not sure how accuruate they are. To be fair I just learned about tidal forces yesterday. The first example that came to mind is a little complicated, but I am in highschool so I assume they can follow my process.

I thought of it as first, a spherical magnet suspended in 0 gravity, which would represent the earth. Then for the liquid aspect, I though that a metalic liquid thats attracted to the spherical magnet. The magnet roatates just like the earth. Then if you bring in another magnet, and it is placed at a distance where it only pulls slightly on the spherical magnet and the liquid metal as it roatates, like the moon. So where the magnet attracts the liquid metal there is a bulge, just like how the moon ceates a bulge with water on earth.

The second one I thought of might be easyer to grasp, but I am not sure how logical or possible it is. I was thinking of the the common experement where you pour water and hold a charged ballon nearby the stream of water, so that the water bends twords the ballon because of the opposite charges. and I thought that, what if you took that principal, and had water in the 0 gravity where it becomes a sphere as you said, and then did the same thing, so the opposite charges would attract, but instead of bending a stream of water, it would create the bulge on the water sphere. For this example however, Im not sure how a solid ball inside the water would effect the outcome. But it is a similar idea in theory.

I belive I understand the question, but if I miss understood please let me know.


Let's start with the tides. The earth and moon both orbit their common center of mass (CM). The center of mass is just below the surface of the earth and follows a smooth elliptical orbit around the sun. These orbs move freely through space subject to mutual gravitational forces. The gravitational field from the moon is stronger on the near side oceans than on the rest of the earth, causing a high tide. It is also stronger on the planet than on the far side ocean. With less centripetal force, the far side ocean tries to follow a larger orbit about the CM (also producing a high tide). Friction with the rotating surface drags these tides East relative to the moon. Since gravity between objects of normal size is quite weak, this situation would be difficult to simulate on the earth. You might consider molten iron in a ceramic bowl which is being moved in a circle around a magnet.


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