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Homework Help: Do I have the right amount of sig digs?

  1. Jan 26, 2015 #1
    1. The problem statement, all variables and given/known data

    To simulate gravity a circular space station with a radius of 150m is rotated so that astronauts on the inner surface move at 30m/s. If a 75kg astronaut stands in a bathroom scale, what reading will it give? In newtons.
    2. Relevant equations
    Fc = mv2/r

    3. The attempt at a solution
    Fc = Fg

    Fg = mv2/r

    Fg = (75)(900)/150

    Fg = 450 N

    Therefore the scale will say 450 N.

    I dont get it... 30m/s is 1 sig dig. So my answer should be 400 N? Can somebody confirm this? It just seems weird to write "Therefore the scale will say 400 N" when in reality it will be 50 N greater, which is a big difference. I have to submit my answers for marking and I dont want to lose marks. So are you supposed to use sig digs even if it changes your answer a lot? Because there have been a few instances where I come across questions that should have 1 significant digit in the final answer but that would change my calculated answer by a lot.
  2. jcsd
  3. Jan 26, 2015 #2


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    Interpreting significant digits from the way numbers are shown is a bit tricky.
    Ideally, it should be 3E1 (3*101) to show 1 sig dig, or 3.0E1 to show two, etc. I would take 30 as showing two.
  4. Jan 26, 2015 #3
    Im going to ask you about another question, just to make sure I got the hang of this.

    The question gives me 20m and 10m and my final answer was 22m. So this should stay as 22m? Because the only other way you could write 10m is 2x10^1. The fact that it is not written this way means the 0 should be considered significant? Thus, my final answer is 22m instead of 20m.
  5. Jan 26, 2015 #4
    And going back to my original question, wouldnt that mean that 450 has 3 significant figures? Because using that same logic, ideally it should be written as 45x10^1
  6. Jan 26, 2015 #5


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    Technically, but it would be a vindictive examiner that faulted you for answering 450.
    (But I would prefer 4.5E2 to 45E1.)
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