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Newtons Third Law of Motion

  1. Feb 12, 2007 #1
    We often have lots of free time on our hands at work when we should be er... working, anyway we use this time to debate questions that none of us are qualified to answer in a 'I'm 110% right B***H' way. So I decided to come here and post a couple of the ones we have been arguing in the hope that someone who knows his s**t can give us a 'I'm 110% right B***H' answer...

    Todays one was...
    'If you drop a glass of water off a building and ensure it falls straight, would the water fall slower than the glass?'


    P.S*Sorry about the title of the thread it has nothing to do with this question! lol
    Last edited: Feb 12, 2007
  2. jcsd
  3. Feb 12, 2007 #2
    Well think about it, under what circumstances would one part fall slower?
  4. Feb 12, 2007 #3
    Err if erm the.... No, lol I have no idea!
    Basicaly my arguement is that the water would lift from the bottom of the glass and therfore fall slower and hit the ground after the glass...

    My mate(Alex) thinks that the air resistance would cause the glass to fall slower which would hold the water from being able to fall faster so the glass would still contain the water when it hits the ground...

    I'd like to point out at this point I know physics like *Keanu Reeves knows Shakespeare
    Last edited: Feb 12, 2007
  5. Feb 12, 2007 #4
    LOL, I don't even know who Keanu Reaves is. :rofl:
  6. Feb 12, 2007 #5
    lol indeed... I meant Reeves! S**T
  7. Feb 12, 2007 #6
    Your friend Alex is more right than you although his statment isn't 100% correct. Close though. It really wouldn't matter if there was air resistance or not for this experiment. On earth (taking out air friction) and anywhere else there is gravity, everything falls at the same rate. If you were to take a 100 pound ball and a 50 pound ball and drop them from the same hight at the same time, they both would hit the ground at the same time. So picture a cup of water in your hand. The water sits at the bottem of the cup. When you drop the cup both the cup and water accelerate towards the earth at the same time.

    To comfirm your experiment just get a paper cup, fill it with water and drop it and try to see what happens.
  8. Feb 12, 2007 #7
    I think one of the best examples i've seen is this one.

    My professor dropped our physic book and a piece of paper at the same time. The book fell to the group the fastest. Then he placed a piece of paper on the book, and they fell at the same rate. Thus :D Victory for physics.
  9. Feb 12, 2007 #8


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    Oops, I dropped my drink.

    A glass of water dropped straight and upright would arrive at the ground as it left the top. How can I know that? Well, from Galileo I know that all objects fall at the same rate, barring air resistance. So without air we know that the glass will fall together. If we add air resistance then we see that the glass gets slowed down to some extent and would fall more slowly but, since the water is confined by the glass and cannot fall faster, they stay together. Right?
  10. Feb 12, 2007 #9
    Well, the glass acts as a barrier against air friction. At the same time, the water can't go faster than the glass. This means that the glass slows down the water, so they both fall at the same time and in harmony.
  11. Feb 13, 2007 #10
    I get the feeling based on how the problem was stated, that air resistance should be taken into account. If the glass falls upright and doesn't rotate, of course the glass will hit the ground with the water still in it.
  12. Feb 16, 2007 #11
    when air resistance is not negligible
    actually the water will be slower than the glass based on surface area and the glass assumed to have greater weight than total weight of water(under circumstance that the glass fall upside down position). and then the water exert force to the inner base of the glass ( explanation through newton's third law)
    if the glass fall in another style ( not up side down) the glass will have more surface area than the water, in this case air resistance for water is zero (hence falling at exactly 9.81 ms^-2). such as the glass will be slower than water. and with around the same reasoning as previous case, we normally see both fall at the same rate.
    sorry my English sux
    Last edited: Feb 16, 2007
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