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B Looking to understand Apollo 15 and density

  1. Aug 6, 2015 #1
    Okay, so this is the website: <Moderator's note: link deleted as it is not a valid reference>

    I found this page as I was looking for a way to compare the assumed gravitational constant of the moon and the acceleration of the hammer/feather from the Apollo 15 experiment video. I have only taken level 1 physics as an undergrad (last semester, none in HS), but there seem to be some problems dealing with the author's assumptions regarding how density works and whether a pitcher of water is a valid "low gravity environment." A quick free-body diagram seems to be helpful, but I am less than confident in my physics abilities. Therefore, I have two quick questions:

    1-What seems to be problematic with this experiment to you (O, thou who doth possess a grander mind than mine own). I hope to test the strength of my intuition and initial guesses with the help of some more highly educated person(s).


    2-Why does the author feel this breaks Newtonian physics? I took a brief look at the blog/site, and the author seems to be trying to enlighten me and the world; my attention span, however, is not great enough to maintain focus with the temptation of better websites (e.g., physicsforums.com), so I failed to see the point.

    Here's hoping I'm not asking anything too silly, and that I'm posting this in the right place. Please forgive me for any glaring or obvious errors. Cheers!
    Last edited by a moderator: Aug 6, 2015
  2. jcsd
  3. Aug 6, 2015 #2


    Staff: Mentor

    It's claiming that the experiment done in a liquid is a valid test of Newtonian gravity as applied to the hammer and feather experiment performed in vacuum (or the Apollo spacecraft returning from the Moon through vacuum). That claim is, to put it bluntly, bogus.

    To quickly explain why it's bogus: objects freely falling in vacuum have one force acting on them: gravity. Objects "falling" inside a liquid have three: gravity, buoyancy, and drag. The acceleration produced by the force of gravity does not depend on the object's mass (because the force is exactly proportional to the mass), which is why the hammer and feather both fall at the same rate in vacuum. But the acceleration produced by the force of buoyancy--or, more precisely, the resultant force after buoyancy and gravity are combined--does depend on the object's mass (because the resultant force is the difference between the weight of the object and the weight of the water the object displaces, so it is not exactly proportional to the object's mass), which is why the thimbles full of sand and rice start out "falling" at different rates through water. This in itself is enough to make the experiment invalid for the claimed purpose. The force of drag comes into play once the objects are moving, and further complicates their motion, making it even more different from motion in vacuum, and therefore making the experiment even more invalid for the claimed purpose.

    Because he has incorrectly understood what Newtonian physics says about the scenario he is testing.
    Last edited: Aug 7, 2015
  4. Aug 6, 2015 #3


    Staff: Mentor

    Since the article linked to in the OP is not a valid reference, this thread is closed.
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