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Ruler+hammer balance: 3 stages

  1. Mar 28, 2015 #1

    yesterday i saw this video on discovery channel.
    <link to video deleted>
    in the documentation it was said that this trick with the three hammers and
    three rulers doesn´t match with actual laws of physics! does anyone can give more
    information about this?
    Last edited by a moderator: Sep 4, 2016
  2. jcsd
  3. Mar 28, 2015 #2
    Hi Newtonfalls,

    Any object will escape falling over an edge as long as its centre of mass is supported. The position of the COM depends on the mass distribution within the object. It looks like the effect of adding the hammer is to alter this distribution. More specifically, it looks like the first hammer is added such that the centre of mass of the ruler-hammer system moves under the table where it is supported. A hammer is a useful object to do this because most of its mass is concentrated in one end. As long as all the hammers are arranged to keep enough mass far enough under the table so that more force pulls down into the table than into empty space, it should be possible to extend the system like in the video.

    The idea of disproving all laws of physics seems like a bit of sensationalism on the part of the Discovery Channel, since I'm sure PF would be abuzz if that really were true :P
  4. Mar 28, 2015 #3


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    With one hammer, you can find the trick everywhere. The extension looks nice, but it is nothing fundamentally new. The second hammer has to be much lighter than the first one, and the third has to be much lighter than the second one, so all rulers stay balanced.

    I changed the thread title. The experiment is well in agreement with physics, it just can look counterintuitive if you don't think about the mass distributions.
  5. Mar 28, 2015 #4

    Doug Huffman

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  6. Mar 28, 2015 #5


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    This demonstrates the same principle in a clearer fashion, where density is more even. It is also much more impressive in my book:
  7. Mar 28, 2015 #6

    Doug Huffman

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    I well remember Balance Goddess when it first appeared. Most impressive.
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