Set up an infinite row of dominos and knock over the first one

[godspeed]

i'm new to this forum and from first glance it seems like a great place to learn. i have a pretty simple question regarding dominos. if one was to set up an infinite row of dominos and knock over the first one, will the dominos fall for eternity, or will the energy decrease over time from loss due to sound and heat until one domino simply was not hit with enough force to knock the next domino over? some of my friends think energy will be lost and because of the conservation of energy that they will at some point stop, and others think that once you knock over one domino gravity will always accelerate the next enough to knock the next domino over so that it will fall indefinitely. what will really happen and why? thanks.

 

[Hurkyl]

The primary energy source for a domino cascade is gravitational potential energy (GPE); every time a domino falls, its GPE is converted into kinetic energy, thus fueling the cascade.

 

[pallidin]

In a domino cascade, not only will the progression occur, it will actually accelerate! The acceleration effect is cause by the fact that the prior domino, now tilted with force, hits the next domino below the top. This angular action increases the speed of the next dominos top relative to the previous one. And so on.

 

[Bob3141592]

Originally posted by godspeed
if one was to set up an infinite row of dominos and knock over the first one, will the dominos fall for eternity, or will the energy decrease over time from loss due to sound and heat until one domino simply was not hit with enough force to knock the next domino over?

The energy in the next falling domino doesn't just come from the energy transferred from the domino that hits it. If that were the case, then the cascade would eventually stop. But each standing domino also contains it's own potential energy, which was given to it by it's being lifted into an unstable equilibrium by whoever set them up. This will most probably be enough to keep the cascade going for eternity.

The kinetic energy of the striking domino only has to have enough force to overcome the threshold energy that trips the domino out of it's vertical unstable equilibrium. How much energy comes from that domino falling over is a functioon of the geometry of the domino and the strength of the gravitational field, which you haven't specified.

 

[Gara]

are you sure it would get faster and faster? cos with enough time would not the line be falling at the speed of light? (asuming they don't break from each inpact of hitting another one)

 

[LURCH]

Originally posted by Gara
are you sure it would get faster and faster? cos with enough time would not the line be falling at the speed of light? (asuming they don't break from each inpact of hitting another one)

No, it would not accelerate continuously. The acceleration to which Pallidin referred also increases the impact between one domino and the next. Sound waves and heat as well as mechanical vibration of the individual dominos will increase, thus decreasing the efficiency of energy transfer from each collission. This loss of efficiency will fairly quickly reach a state of equalibrium with the enrgy being fed into the system, resulting in a constant speed.

BTW, in case you're wondering where the energy originally entered the system to make this appearance of perpetual motion, the gravitational potential energy was stored in the system when the dominos were first being set up.