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What is the approximate weight of a 20.kg rock

  1. Feb 1, 2012 #1
    Since it has a decimal it would be 20kg * 9.8m/s which equals 196kg m/s^2 = 196N

    But the decimal means i have to round two sig figs, which would make it 200N?

    A very simple problem, I really don't know why I'm asking it.
     
  2. jcsd
  3. Feb 1, 2012 #2

    tiny-tim

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  4. Feb 1, 2012 #3
    Thanks for the reply, tim. "And let me say, that is a big post count"

    My paper says 20.kg so I'm rounding to two significant figures. Am I right?
     
  5. Feb 1, 2012 #4

    tiny-tim

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    i've no idea what "20.kg" means :redface:

    i understand "20 kg" and "20.0 kg", but i've never seen "20.kg" or even "20. kg" :confused:
     
  6. Feb 1, 2012 #5

    Pengwuino

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    Yes that is correct. When you see things like 20.kg, it means that the 0 is significant. If it were simply 20kg without the decimal, that would mean the number is only accurate to 1 significant figure. If, as tiny-tim advised, it were 20.0 kg, it would mean the second 0 is significant as well and there are 3 significant figures to take into account.

    So your answer is 200N, accurate to 2 significant figures.

    Scientific notation is really great because it clears these kind of odd notations. 20kg to 2 significant figures in SI notation would be 2.0 x 10^1 kg, clearly showing the 0 is significant.
     
  7. Feb 1, 2012 #6

    tiny-tim

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    ah! thanks, Pengwuino! :smile:
     
  8. Feb 1, 2012 #7
    Thanks, I knew something was going wrong. My teacher usually does things like that.
    I can't believe sometimes he even gets about 3-6 problems incorrectly which demonstrates unprofessional work ethic.
     
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