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Free fall acceleration

  1. Apr 20, 2013 #1
    hi,

    thanks for taking time to read my first post.

    i'm trying to solve a little mystery and would like to do so in a methodical and logical way.

    it is my basic understanding that free-fall acceleration is a constant on earth of 9.8m/sec2
    that if an object is released from a height it will go through a period of free-fall acceleration before it hits something.

    i assume that it really needs to be in a vacuum to prevent air/wind resistance.

    depending on the answers, i have some further questions.

    thanks again and have a great day
     
  2. jcsd
  3. Apr 20, 2013 #2

    A.T.

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    It is only approximately constant for small fall heights. Outside of Earth It falls off with the square of the distance to the center.


    Yes.
     
  4. Apr 20, 2013 #3
    Thank you ever so much for the extremely prompt reply.

    It's obvious to me from your answer that your physics knowledge is far superior to mine, so please excuse my ignorance.

    Anyway back to the mystery.

    If a free-falling object was known and accepted to have gone through a period of free-fall acceleration, is it correct to say that it cannot have encountered any resistance during that period?

    Thanks again.
     
  5. Apr 20, 2013 #4
    Your curiosity prompted me to ask the following question:
    If you can add mass to an object at a rate that object experiences acceleration, then does that object move?
    Feel free to ridicule. The question pop up out of no where without suppressing the urge to post it without giving it further thought.
     
  6. Apr 20, 2013 #5
    If the initial velocity was 0 then yes. But if it's been pushed towards ground, then there might have been some resistance, if I understand your setup correctly.

    @Hush:
    Yes, the object moves, unless there is a friction.
     
  7. Apr 20, 2013 #6
    Thanks again to everyone for your responses.

    This is exactly how I wanted to solve the mystery and establish the facts.

    Yes the initial velocity is 0. It's not being pushed or pulled towards ground, unless the sheer weight of an object can change the velocity? I seem to remember that a feather and a coin fall at the same velocity in a vacuum.

    could the creation of a vacuum under the object 'suck' the object downwards faster than free-fall?
     
  8. Apr 20, 2013 #7

    sophiecentaur

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    It is the weight that causes it to accelerate downwards when it falls.

    If the pressure is reduced 'below' an object, the excess pressure above it will increase its rate of fall. The experiment with the hammer and the feather was conducted in the absence of any air around them (i.e. on the Moon).
     
  9. Apr 21, 2013 #8
    Thanks again for the replies.

    To expand upon the mystery, the free-falling object is known and accepted to have gone through a period of free-fall acceleration, yet there was, or appeared to be, something of considerable mass and rigidity directly in it's path to Earth.

    Would it be correct to hypothesize that the obstacle(s) must have been removed at some point, measured in nanoseconds, between the initial velocity of 0 and the period of free-fall acceleration?
     
  10. Apr 22, 2013 #9
    Try this:
    Cut out a piece of paper smaller than the face of a large coin.
    Hold each in your two hands and drop them.
    Note the result.
    Now, lay the paper on the top of the coin and carefully drop the coin so that upper side stays facing up.
    The coin drops the same way as before... but now the paper drops... how?
    Now do it again with the paper on top of the coin, but this time, instead of dropping the coin, pull the coin down faster than its drop speed... now what happens to the paper?

    This should give you some insight (and probably more questions) about your question.
     
  11. Apr 23, 2013 #10
    Once again I thank you all for your replies. We need to get to the bottom of this mystery as it is really quite important and I don't say that lightly. We're nearly there, I just need to qualify a couple more points before revealing the whole mystery.

    Apologies for repeating myself:

    To expand upon the mystery, the free-falling object is known and accepted to have gone through a period of free-fall acceleration, yet there was, or appeared to be, something of considerable mass and rigidity directly in it's path to Earth.

    Would it be correct to hypothesize that the obstacle(s) must have been removed at some point, measured in nanoseconds, between the initial velocity of 0 and the period of free-fall acceleration?

    new addition i.e. the obstacle would have to be removed at or above the rate of free-fall acceleration.
     
  12. Apr 24, 2013 #11
    The object is within a couple of hundred metres of Earth.

    To use an analogy, it‘s like a book is sitting on a table or perhaps more accurately a book sitting on a pile of books.

    It then appears to fall straight through the table or pile of books, going through a period of freefall acceleration, before hitting the ground.

    So the mystery is how does the book fall?
    Surely the table or pile of books would need to be removed at or above the rate of freefall acceleration?
    If that is so, then how was the obstacle(s) removed and so quickly.
     
  13. Apr 24, 2013 #12

    Doc Al

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    Why in the world would you think that the "obstacle" must be removed at some particular rate?

    Just hold a book in your hand and let it go. As soon as the the book is completely free of your hand, it is in free fall. (Neglecting air resistance, once again.)
     
  14. Apr 24, 2013 #13
    hypothetical question

    stop asking hypothetical questions. the question of what would happen if such and such existed is hypothetical and irrelevant. if you have a real life example which you are trying to understand then you can mention that.
     
  15. Apr 24, 2013 #14

    Dale

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    This is a self contradiction. If you KNOW that an object went through a period of free fall acceleration then you KNOW there was nothing of any rigidity directly in its path, by definition.

    It seems like you may be on the edge of proposing something speculative and contrary to established science. I recommend that you review the forum rules before doing so.
     
  16. Apr 25, 2013 #15
    To clarify, as initially stated I started this thread in an attempt to get to the bottom of a physics mystery.

    I'd chosen to obscure the true nature of the mystery due to the distinct possibility that the ramifications would interfere with the confirmation of established science.

    I was 99.9999% sure that I already knew the answer but I wanted to confirm my suspicions with physics experts.

    So far we have reached the conclusion that an object with an initial velocity of 0 which is known and accepted to have gone through a period of free fall acceleration within 200 metres of Earth could not have had anything of any rigidity directly in it's path, by definition. And therefore anything apparently in it's path must have been removed prior to descent initiation.

    The object in question is a 47 floor modern steel framed building. Construction started 2002 and it opened in 2006. It collapsed in about 7 seconds. It is known and officially/scientifically accepted to have gone through a period of free fall acceleration.There are multiple videos of the collapse available.

    Given the debate here I assume that it is still accepted that the floors below must have been removed at or above the rate of free fall acceleration in order for the entire structure to collapse into a pile of rubble and steel beams.

    I realise that some of you may know exactly what building I am talking about and some of you may not.
    The ramifications of this mystery are enormous. All I ask is that you keep an open mind and attempt to suspend your belief system in order to let the scientific facts speak for themselves.
     
    Last edited: Apr 25, 2013
  17. Apr 25, 2013 #16

    sophiecentaur

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    This is nonsense, I'm afraid and there is no "mystery" involved. The detailed Maths may be a bit on the hard side but that's not a mystery - it's the same for all Engineering problems.
    A building which collapses in this way is not in any kind of free fall. It is subject to a series of resistive forces as it falls. Gravity is accelerating the top bits downward and they are impacting on and collapsing the lower storeys - one at a time and starting at the bottom. All that's necessary is for the velocity to increase at a rate that is sufficient to compensate for the decreasing falling mass so that there is enough energy available to crush the lower supports.

    If you had introduced the question more explicitly in the first place, we could have sorted things out much earlier. The essence of a good solution is a well stated question.
     
  18. Apr 25, 2013 #17
    After a lengthy investigation by a respected government department tasked with scientific investigation they have stated that it went through a period of free fall acceleration.

    sophiecentaur:
    I'm not clear from your reply if you suggesting that the inertia of the collapsing building was enough to crush the lower supports fast enough that it would appear to go through a period of free fall acceleration.
    Surely the "series of resistive forces" would have slowed the descent considerably, as the initial velocity was 0 and there was no external downward force, apart from gravity, acting upon it.

    I sincerely apologise for any confusion or annoyance but I felt I had to address the issue in this way.
     
  19. Apr 25, 2013 #18

    sophiecentaur

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    Free fall: That means with no retarding forces, which couldn't have been the case. 'Effectively Free Fall' might be a reasonable phrase and this would have taken place when the pillars of the ground floor gave way. It would need to have been either the weakest layer of pillars or the layer with the biggest load - so I assume it would be the ground floor that gave way. The whole of the rest of the building would have fallen by, say 3m, giving it a downward speed of about 7m/s (round figure). The impulsive load on the next supporting layer would have been more than the static load which broke the first layer of pillars so the next layer would probably have collapsed without slowing the fall by much. Then the building would have fallen a further 3m, accelerating further until the next impact. Less mass now, of course, because two floors were on the ground but more speed, so enough to break the next layer of pillars.
    For the whole building to collapse, all that would be necessary is for the remaining sections to have reached a high enough speed so that bringing mass mass to a halt would involve enough force to break each successive layer of pillars.

    The only 'mystery' in this scenario is how any structural engineer would not have specified dimensions and materials for the lowest level to support (with loads to spare) the whole of the building at all stages of construction. I would look for conspiracy (corruption) here. There is a lot of money to be made by anyone unscrupulous enough to use lower specified construction materials in a big project and you only need to pay off any inspectors or potential whistle blowers. Or it is always possible that some incompetent Engineer is used, who got his post through nepotism or other corruption. Most times, you get away with it but there are regular reports of buildings spontaneously collapsing in different parts of the world.
    It is notable that it doesn't tend to happen in Europe or the US. I wonder why that would be???
    In the UK, building regulations specify that even a single storey building must have foundations which are suitable to take more floors, just in case someone might want to extend upwards. That's an awful lot more concrete than absolutely necessary, taken over the whole country - but we can sleep easy in our beds as a result.
     
  20. Apr 25, 2013 #19

    Doc Al

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    Spoken like a true conspiracy theorist.

    Please reread sophiecentaur's posts.

    Thread closed.
     
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