Great Video BC & RD. I might have to find a set of dominoes to play with, or perhaps makes some pseudo-dominoes.

Somebody has probably worked out all the domino equations.

The tipping point of the domino is where the center of gravity is exactly over the corner, or where the two opposite corners line up vertically. That also delineates the energy required to lift the center of mass to the tipping point.

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I believe the minimum spacing for equal dominoes to sustain a chain reaction would be where the center of mass of the falling domino is at the same elevation of the center of mass of the standing domino (where it was). Is this twice the first angle to the tipping point? This would be slightly less than twice the thickness of the domino. Or, somewhere thereabouts.

Is spacing them a single domino thickness apart too close?

Of course, all the points are moving, so as the next domino topples, the previous one is still pushing slightly, so you may be able to space them closer than I had originally guessed, but still, a single domino width is probably close to the minimum.

I'm not quite sure what the optimal spacing is, and it may well be different for equal sized dominoes, and different sized dominoes.

However, there may be some acceleration of toppling of equal sized dominoes.

So, the film suggested about 1.5X for successive larger dominoes.

ABCDE

Now, what if one put multiples in?

AAA BBB CCC DDD E

Would one experience slight acceleration of the toppling of identical dominoes, and thus being able to topple slightly larger dominoes, perhaps 1.6 or 1.7X?