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Here's a gif showing how the balls move when I move the rattle. The circular tube hangs vertically, with the balls on the bottom. There are more images in the bottom.

enter image description here

  • The balls roll freely inside the tube
  • The inner diameter of the tube is larger than the diameter of the balls
  • I have tried taking an external magnet close to it, but the balls aren't affected by it.

What makes these balls repel each other?

enter image description here

As you can see from the image below, the diameter of the balls is smaller than the inner diameter of the tube (it is identical if I flip it).

enter image description here

Edit: Here's a gif showing what happens if I leave it be for a while, then shake it. Looks like knzhou is right.

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    $\begingroup$ This sounds like a great exercise for students when teaching physics at school to introduce electricity and static charges. $\endgroup$
    – Gimli
    Commented Mar 22, 2018 at 18:43
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    $\begingroup$ Can you tell us the brand of toy this is? I want to buy one to illustrate static electricity to students. $\endgroup$
    – Yly
    Commented Mar 22, 2018 at 19:49
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    $\begingroup$ I wish I could. There are no indications of brand/name or anything else on it. It's not new, so I can't recall where it's bought either. $\endgroup$
    – Stewie Griffin
    Commented Mar 22, 2018 at 19:56
  • $\begingroup$ @Yly Go to any store that sells baby toys. This is a very common theme in rattles. There are many different brands that use the same idea. $\endgroup$
    – Selene Routley
    Commented Mar 22, 2018 at 23:27
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    $\begingroup$ I believe it's a custom version of a rattle manufactured by Tomni Design (AU). They have 21 variations visually similar to this rattle (minimum purchase =1), although none with only 4 balls. I emailed the company for confirmation, directing them to this thread. Who knows, this discussion could open up a whole new demographic for them. :-) $\endgroup$
    – ashleedawg
    Commented Mar 26, 2018 at 11:30

3 Answers 3

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As you said, it's probably not magnetism if the balls are free to rotate; there is no reason they wouldn't just flip over and stick together, north to south. You can test this by buying some of those toy magnetic balls. The repulsive configurations are highly unstable and turn attractive with the slightest touch.

I'm going to go out on a limb and say it's static electricity; the balls are picking up charge by rubbing against the plastic. The electrons will always go to whichever material 'pulls' them harder (according to the triboelectric series), so the balls all get the same charge and hence repel.

I don't have a baby rattle with me but here are some ways you could test it:

  • If you don't move the rattle for a while, the balls should come together as the static dissipates
  • The effect should come back once you shake the rattle a couple times
  • The effect should be smaller on humid days where static dissipates faster
  • Another statically charged object should attract/repel the balls, e.g. a balloon rubbed on hair. (The plastic ring will not block this effect, since it's an insulator.)
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    $\begingroup$ If the air inside the rattle is sealed off from the atmosphere, the effect might not change much on humid days. $\endgroup$
    – Michael Seifert
    Commented Mar 22, 2018 at 14:21
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    $\begingroup$ You are very likely right :) $\endgroup$
    – Stewie Griffin
    Commented Mar 22, 2018 at 14:24
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    $\begingroup$ It seems like a reasonable hypothesis. If you have access to a radioactive source (an Xray tube would be even better... easy to get high flux), you could irradiate the rattle. If the balls move closer together, your hypothesis is confirmed (ionizing the air would speed up the discharge process). $\endgroup$
    – Floris
    Commented Mar 22, 2018 at 19:57
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There aren't that many repulsive forces out there which could be at play:

  • magnetism
  • electrostatic forces
  • compressed material (solid, fluid or gas) pushing bodies apart

Magnetism would have worked if the bodies would not be able to rotate, or if we could produce magnetic monopoles.

Compressed springs are extremely hard to make completely invisible. Liquid or gas could be unnoticeable, but like you said, those balls don't seal the tube and cannot prevent the gas from escaping.

Which leaves you with pretty much the only option: electrostatic forces.

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    $\begingroup$ Or black magic :-) $\endgroup$
    – user89712
    Commented Mar 26, 2018 at 2:02
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The balls are what's called electrets. They retain a static electric charge that is uniform within them. This is the same effect makes plastic cling wrap work. You can look up "electret" on the web to find out more.

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  • $\begingroup$ The reason that they repel is that they all have the same polarity. $\endgroup$
    – Charles Crummer
    Commented Mar 27, 2018 at 19:35

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