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Homework Help: Universal gravitation 7- determine the gravitational force of attraction

  1. Jun 15, 2012 #1
    1. The problem statement, all variables and given/known data

    Michael has a mass of 75.0 kg and Elaine has a mass of 55.0 kg. If Michael and Elaine are 2.50 m apart from each other as they sit in their Physics class, determine the gravitational force of attraction between them.

    2. Relevant equations
    I have made a list of equations that are relevant for this entire module on universal gravitation. So although there are many of them does not mean that they all apply in this circumstance. The ones relevant to this question will be placed in bold.

    Kepler's 3rd law: (Ta/Tb)2=(Ra/Rb)3

    motion of planets must conform to circular motion equation: Fc=4∏2mR/T2

    From Kepler's 3rd law: R3/T2=K or T2=R3/K

    Gravitational force of attraction between the sun and its orbiting planets: F=(4∏2Ks)*m/R2=Gmsm/R2

    Gravitational force of attraction between the Earth and its orbiting satelittes: F=(4∏2Ke)m/R2=Gmem/R2

    Newton's Universal Law of Gravitation: F=Gm1m2/d2

    value of universal gravitation constant is: G=6.67x10-11N*m2/kg2

    weight of object on or near Earth: weight=Fg=mog, where g=9.8 N/kg


    determine the mass of the Earth: me=g(Re)2/G

    speed of satellite as it orbits the Earth: v=√GMe/R, where R=Re+h

    period of the Earth-orbiting satellite: T=2∏√R3/GMe

    Field strength in units N/kg: g=F/m

    Determine mass of planet when given orbital period and mean orbital radius: Mp=4∏2Rp3/GTp2

    3. The attempt at a solution
    So we know the following
    m2=55 kg
    d=2.5 m
    G=6.67x10-11 N*m2/kg2

    So with the equation highlighted above I solved for F=4.4022x10-8N

    I would love for someone to verify my work done on this problem and let me know if i made any mistakes and point them out to me, it would be greatly appreciated! Thanks so much in advance :)
  2. jcsd
  3. Jun 15, 2012 #2
    Your work is correct, again.

    PS : You don't need to post all obviously simple questions to verify. Be confident that you've done 'em right! :wink:
  4. Jun 15, 2012 #3
    thanks! I just feel better knowing that Im doing it right, taking physics through correspondence isn't always easy haha
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