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In this video a single flick of a finger tips 116000 dominos.

Domino video

I understand the work that needs to be done to move 116000 pieces (at least 100 kilos) of plastic is greater then that expended by flick of a finger. How does the energy conservation law apply here?

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When setting up the dominos, you rise their tops against the force of gravity, thus they gain potential energy. If you tip them over, they release that energy. –  Lagerbaer Jul 26 '13 at 20:51
@Lagerbaer Why not elaborate a slightly and turn your comment into an answer? –  joshphysics Jul 26 '13 at 20:55
I think Lagerbaer and udiboy are right. it's kinda like dropping a ball from a hill and saying that I only moved my fingers a bit. –  mehfoos Jul 26 '13 at 21:05
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1 Answer

up vote 8 down vote accepted

Gravity is doing that work!

If you observe, the domino is in a position of unstable equilibrium.

Edit: as pointed out in the comments, this position is of a metastable and not unstable equilibrium.

This means that the domino is in a state where it hasn't achieved the minimum possible energy state yet. The energy I'm talking about here is the gravitational potential energy.

Gravitational potential energy is directly proportional to the height of the center of mass from some reference point.

That means that when the domino is standing up it has more energy than when it is laying flat. So when you flick the domino, it tends to fall down. It is just trying to achieve a state of lower potential energy.

Now when you have a lot of dominoes standing up in a row, the whole system of dominoes can be regarded to be in a position of unstable equilibrium(metastable equilibrium), i.e. their combined center of mass is at a greater height than when every one of the dominoes has fallen. So all you need to do is flick one domino and they will all come tumbling down.

You can also think of it this way.

The people who arranged those dominoes initially had to do work against gravity in arranging each domino in the upright position.
So they had to provide the whole system of dominoes with energy initially, which was then all dissipated due to work done by gravity after the flick.

So energy is still being conserved!

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Sorry for the late answer. I was busy watching those dominoes fall! –  udiboy Jul 26 '13 at 20:59
Good answer! One small suggestion: The dominoes are in a state of metastable equilibrium. A small disturbance would cause a slight rocking, followed by a return to its on-end position. A needle balanced on its point is an example of unstable equilibrium. –  User58220 Jul 28 '13 at 2:58
@User58220, I had exactly that same doubt. The unstable equilibrium position should be when it is balancing on one edge! Thanks for clarifying it! –  udiboy Jul 28 '13 at 10:52
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