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Homework Help: Algebra Question

  1. Jul 22, 2015 #1
    1. The problem statement, all variables and given/known data
    Find Fg if G=6.67*10-11m3 kg-1s-2, M=2.6*1023 kg, m=1200kg, and r = 2000m

    2. Relevant equations
    Fg= -GMm/r2

    3. The attempt at a solution

    Fg=- (6.67*10-11m3kg-1s-2)(2.6*1023 kg)(1200kg)/(2000m)2

    Fg=- (6.67*10-11m3kg-1s-2)(2.6*1023 kg)(1200kg)/4000000 m2

    Fg= - 2.08*1016 kg m3 s-2/4000000m2

    Fg= - 5,202,600,000 mkg/s2

    Fg= - 5.21*109 mkg/s2

    First, I'm not sure how to deal with the units here. I added the exponents across and subtracted if they had the same unit below them but I'm not sure I ended up with the right units. Also, I don't know if you can have a negative number in scientific notation. Lastly, I don't even know if the problem was solved correctly.

    Thank you for any help, I'm beyond rusty at math like this.
  2. jcsd
  3. Jul 22, 2015 #2


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    Look like a unit of force to you?
    Can you think of any reason there shouldn't be negative numbers?
  4. Jul 22, 2015 #3
    I would think it best to leave the units out of the calculations . The formula will eventually work out to give you the Unit of force - Newton(N) .

    Yes you can . Scientific notation only helps to make working with large numbers easier - It doesn't have any relation with the sign of a number . -120000 = -1.2*105 .

    Also , you seem to have worked out your answer probably .

    Hope this helps .
  5. Jul 25, 2015 #4
    No! Never leave out the units! Aside from needing units to be formally correct, the units provide a very neat way of checking your answers.
  6. Jul 25, 2015 #5
    Just to be clear , I was talking about using units during steps .

    However , if this is what you intended to reply to , point well taken .
  7. Jul 25, 2015 #6


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    I am puzzled as to why this was titled "Algebra Question". Other than the "physics" involved in the given formula, this is purely arithmetic.
  8. Jul 26, 2015 #7
    Your units are correct. The unit for Force is a Newton, which is defined as one kilogram times one meter per second squared (mass times acceleration).
  9. Jul 26, 2015 #8
    An interesting problem. Realistically though, what could that massive object be?
  10. Jul 26, 2015 #9


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    It's not the mass in itself that's remarkable, it's less massive than the earth. The difficulty is combining that with a distance of 2000m.
    Strictly speaking, there is not enough information. We have to assume that M is a sphere composed of concentric uniform spherical shells, with an outer radius less than 2000m.
    Maybe there was more text in the original question.
  11. Jul 26, 2015 #10
    My first thought was "neutron star" but, although the density is right, it doesn't have enough total mass to be one.
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