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I'm trying to model/qualitatively analyse the velocity components of a toppling domino about a pivot point, to support the results I procured in my experiment.

Essentially I am trying to figure out why a domino that is falling through a small angle has a slower velocity than a domino falling through a medium angle...about 45°. And why a domino falling through a big angle 45°< $\theta$ < 90° is also slower than the medium angle.

enter image description here

That was the diagram I made for the above description. However, I think that everything explained above is not technically correct.

Edit

I am looking at how the spacing (spacing changes the size of the angle the domino can fall through) between 2 dominoes affects the overall speed of the wave. Here is a graph of some results: enter image description here

I am not sure how the forces change to produce this.

sammy gerbil
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Gypsy
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  • have a look at this, it might be helpful: https://ysjournal.com/archive/issue-5-1/measuring-the-velocity-of-toppling-dominoes/ –  Feb 09 '18 at 22:51
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    @countto10 I have seen this :) However, it only discusses the velocity change respective to the angle of the surface, not how the velocity is impacted by the angle through which the domino pivots. – Gypsy Feb 09 '18 at 23:23
  • A small suggestion. Your question would be improved (OK, I really mean noticed more :) by a small illustration, with a free body diagram. Also, put up your experimental data. How did you time events? +1 –  Feb 10 '18 at 01:03
  • I do not fully understand this part: "but since there is no vertical movement of the domino, the velocity stays the same, whilst the distance to fall through increases" Why do you say that there is no vertical movement? And no movement does not mean constant velocity, it means zero velocity. Is there a typo somewhere here? – Steeven Feb 10 '18 at 11:11
  • @Steeven I don't understand it either, that was what my friend said.... (The whole explanation seems wrong to me) – Gypsy Feb 10 '18 at 11:48
  • Thank you for posting a graph. When you say "speed of the wave" I think you mean the average speed. Is that correct? ie The length of the domino chain divided by the time between the 1st domino being pushed and the last domino falling. And is it correct that your domino chain is on a horizontal surface, not an inclined surface? – sammy gerbil Feb 11 '18 at 02:03
  • @sammygerbil Yes that is all correct – Gypsy Feb 11 '18 at 02:20
  • How long was the domino chain? Did you keep the same number of dominoes in the chain when you increased the spacing, or did you keep the total length of the chain constant? Did you start timing when you pushed the 1st domino, or later, eg when the 10th domino started falling? Did you do the timing with a stopwatch or did you use a light gate? What are the dimensions of the dominoes (height vs thickness)? – sammy gerbil Feb 11 '18 at 02:27
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    The chain was 25 dominoes long in each case, I recorded it with a high speed camera. The end result was the time taken for the last 19 dominoes to fall. Dimensions are approx. 6:3:1 (Also, should add that in order to calculate the velocity, I ended up diving the pitch of one domino by the av. time it took for one domino to collide with the next) – Gypsy Feb 11 '18 at 02:32
  • It is entirely possible that for large spacings the next domino is not going to pivot, but slide. Or do a combination of pivot and sliding. Ideally the impact point needs to be on the axis of percussion. For a block this point is $\frac{2}{3}$ the way up. – John Alexiou Feb 11 '18 at 02:57
  • @ja72 So if the the domino impacts closer to the axis of percussion, it exerts a maximum force (never really come across AoP, so not sure what it does), and consequent velocity would be greater? (Risk of sliding was largely negated by the use of a sandpapered surface) – Gypsy Feb 11 '18 at 03:03
  • @Gypsy - if it impacts near the center of mass then the momentum transfer is going to be %100 translational. If the domino tips, that would be the result of friction, and not of it being knocked over. – John Alexiou Feb 11 '18 at 16:50
  • @Gypsy Can you identify sliding in your video? Are you able to upload it to YouTube and post a link here? – sammy gerbil Feb 11 '18 at 20:44

1 Answers1

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A falling domino has rotational acceleration of $$\ddot{\theta} = \frac{3 g}{2 \sqrt{h^2 + w^2}} \sin \left( \theta - \tan^{-1} \left( \tfrac{w}{h} \right) \right) $$

where $g$ is gravity, $h$ is the domino height, $w$ is the domino width and $\theta$ the tilt angle. The above is solved in terms of rotational velocity if one assumes the initial rotational velocity (after impact) is $\omega$

$$ \dot{\theta}^2 = \omega^2 + \frac{3 g}{h} \frac{h \sqrt{h^2+w^2}-w (s-w)-h \sqrt{h^2-(s-w)^2}}{h^2+w^2} $$

where $s$ is the separation between dominos (not the gap, but the distance between the centers).

The trick now is for the falling domino with $\dot{\theta}$ to impact an upright one resulting in speed $\omega$. To find this, one needs the reduced mass $\mu$ of the system along the contact normal. If each domino has mass $m$, then the reduced mass is

$$ \frac{ \mu}{m} = \tfrac{h^2 (h^2+w^2)}{44 h^4 + 3 w^2 (s-w)^2 - h^2 (39 s^2-78 s w + 37 w^2)+6 h \sqrt{h^2-(s-w)^2}(2 h^2 +3 w (s-w))} $$

The speed of the contact point being $v_{impact} =\dot{\theta} \sqrt{h^2 - (s-w)^2}$ and the exchanged momentum $J = \mu\, v_{impact}$ (assuming coefficient of restitution equals 0)

As a result, the initial (rotational) speed of the next domino after impact is

$$ \omega = \frac{ \dot{\theta} 6 h^2}{h^2+w^2} \tfrac{ h (h^2+w^2) \sqrt{h^2-(s-w)^2}+2 h^4+2 h^2 s (2 w-s)-2 w^2 (s-w)^2}{44 h^2+3 w^2 (s-w)^2-h^2 (39 a^2-78 s w + 37 w^2) + 6 h \sqrt{h^2-(s-w)^2} (2 h^2 +3 w (s-w))}$$

Combine the 2nd with the 4th expression from above to solve for the initial rotational speed of the domino

$$ \omega^2 = \frac{ 1 - \tfrac{(h^2+w^2)^2 \left(6 h \sqrt{h^2-(s-w)^2}(2 h^2+3 w (s-w))+44 h^2 - h^2 (39 s^2-78 s w+37 w^2)+3 w^2 (s-w)^2\right)^2}{36 h^4 \left(h (h^2+w^2) \sqrt{h^2-(s-w)^2}+2 h^4+2 h^2 s (2 w-s) -2 w^2 (s-w)^2 \right)^2}}{\tfrac{3 g (h \sqrt{h^2-(s-w)^2}-h \sqrt{h^2+w^2}+w (s-w)}{h (h^2+w^2)}} $$


Reference: Calculate force between rotating objects and Impulsive angular momentum.

John Alexiou
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  • Are you able to relate this to Gypsy's graph? – sammy gerbil Feb 15 '18 at 11:09
  • I can't because there isn't an analytical expression for the speed or the time to impact. To get to time I need to solve an elliptic integral, which I can't do unless I have more specific details of the problem at hand. You can hower estimate that higher rotational speed after impact reduces the time to the next impact. But with higher separation, the time to impact should increase significantly. Overall this is a very non-linear problem with a lot of subtle effects at play. – John Alexiou Feb 15 '18 at 16:27