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B Time of free fall does depend on mass

  1. Sep 17, 2016 #1
    Months ago I have seen walter lewins first lecture on dimensions, there he made a comment that scientist are trying to prove that time taken to reach ground when a object is dropped does depends on mass (in small amount) but it depends on mass, how they have thaught that it depends on mass , formula says T = C√(h/g) and T is only proportional to height or distance , then how there can be a relation between mass and time, what made them to make a research on this, answer from a researcher in this field will be great.
     
  2. jcsd
  3. Sep 17, 2016 #2

    CWatters

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    Im a bit rusty but this sounds like you are referring to the possibility that gravitational and inertial mass could be different.

    For example we can write...

    GMm/r^2 = ma

    Where m on the left is the gravitational mass and m on the right is the inertial mass. Normally we assume these are the same so the mass m cancels. That makes the acceleration a independent of m. But what if it's not exactly the same.
     
  4. Sep 17, 2016 #3
    Okk , wohoo you made me think a lot , how can we say that a same particle can have two masses.
     
  5. Sep 17, 2016 #4
    Why wouldn't it?
    An incorrect assumption is that the only one of the masses under mutual gravitational.
    Both masses move towards one another.
     
  6. Sep 17, 2016 #5

    Vanadium 50

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    This is guesswork, and it's because the OP didn't ask a good question.

    A good question: "In the video at this link, at 4:58, Prof. X says Y. Can you explain how Y and Z can both be true?"
    A bad question: "In one of his videos, Prof. X says something that might have been Y. I don't understand/"
     
  7. Sep 17, 2016 #6
    It is generally expected that inertial and gravitational mass are equivalent. The reason people are looking for a difference is because that would be big news. It's human nature to want to set yourself apart from the crowd. What better way to do that then proving that physics is radically different then everyone thought? So, people keep rechecking old theories with ever greater and greater acuracy, hoping to prove them wrong and find new physics. Sometimes, very rarely, it works, and progress is made. :-)
     
  8. Sep 17, 2016 #7

    sophiecentaur

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    This classical treatment may be all that's involved. The OP doesn't make it clear.
    Can the OP give a link to the particular lecture that they are quoting from?
     
  9. Sep 17, 2016 #8
    , go to this link and you will get walter lewin's first lecture , he discusses all this at the end of video
     
  10. Sep 17, 2016 #9
    time :36.16 minutes
     
  11. Sep 17, 2016 #10

    Vanadium 50

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    Lewin says that people are checking on what we think we know by performing ever more sensitive experiments.
     
  12. Sep 17, 2016 #11
    Why they are checking , formula simply shows that time depends on height only then why do we check for mass
     
  13. Sep 17, 2016 #12

    Vanadium 50

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    Because physics is an experimental science, and we want to verify that the formulas we use are correct to the best of our ability.
     
  14. Sep 17, 2016 #13
    Ok how can you say that they are only verifying their formula
     
  15. Sep 17, 2016 #14
    In Newtonian mechanics (two body problem):

    The universality of free fall is always true. The time of free fall is affected by the mass of both bodies.

    The acceleration of body A is affected only by the mass of body B, and the acceleration of body B is affected only by the mass of body A.

    If the mass of free falling body A is changed, it's acceleration will not change, but the time of free fall will change.

    If I'm understanding the op post correctly you may find the above statements confusing or even contradictory, but they are all true. I puzzled over this for a long time. It's one of those things where once you get it, it will be an aha moment. Until you understand why the above statements are all true, I think it would be unwise to try and understand the concepts of active, passive, and inertial mass.
     
  16. Sep 17, 2016 #15

    Vanadium 50

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    I could repeat what I said, only louder, but I don't think it would help.
     
  17. Sep 18, 2016 #16

    sophiecentaur

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    What are you trying to say? You seem to be suggesting 'something' is wrong but you are not specifying just what is wrong.
     
  18. Sep 18, 2016 #17

    David Lewis

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    In General Relativity, a force is felt when a body accelerates in spacetime, suggesting inertial and gravitational mass might be two ways of looking at the same thing.
     
  19. Sep 18, 2016 #18
    The formula that you are referring to has limitations. It is usually used for measurements near the earth surface where the value of h is limited. g is not necessarily a constant. It changes with a change in the value of h, thus the limitation. In addition to the value of g being affected by the value of h, it is also affected by the value of the combined masses of both bodies. So to get a more precise value for T we should check for mass also.
     
  20. Sep 18, 2016 #19

    sophiecentaur

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    Because that's the way that Physics is done. If there is no experimental evidence (and that includes finding that something is NOT a factor) then a theory is not acceptable.
    Dimensional Analysis is very powerful at generating ideas and theories but it is not enough because the formula that it's applied to may not actually be correct.
     
  21. Sep 18, 2016 #20
    A more accurate equation for free fall time can be derived from Kepler's third law:
    gif.gif
    This equation has the advantage that it accounts for both the distance R and the masses M and m. But it also has the disadvantage that M and m are considered to be point masses. So you would need to account for the physical size of the bodies.
     
    Last edited: Sep 18, 2016
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