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Homework Help: Moment of inertia and velocity of an object

  1. Mar 21, 2009 #1
    1. The problem statement, all variables and given/known data

    1.)A regular hexagon with sides of length 7 cm has a point mass of 1 kg at each
    vertex. What is the moment of inertia for rotation about an axis which goes through the
    center of the hexagon, and is perpendicular to the plane of the hexagon? Note that the
    sides of the hexagon are made of rods with negligible mass

    2.) The mass of the moon Io is 8.93x10^22 kg. Let 1.82x10^6 m be RI which is the radius of Jupiter’s moon Io. If there were a small asteroid traveling in a circular orbit around Io at a distance of 2RI above Io’s surface, what would be its speed?

    2. Relevant equations

    1.I=ICM+MD^2 = I= (M(L^2))/12 +(M(radical3/2(L))^2)

    2.) v= sqrt(GM/r)

    3. The attempt at a solution
    1.) I plugged in M=1 and L=.07m and ig to teh radical 3/2 since that is the length of side that is parallel to the side of the hexagon but when i get my final answer it was the wrong answer where did i go wrong...is it the the radical 3/2?

    2.) I plugged in the values for G and M as well as 3 times the radius of the moon since the small asteroid is twice the distance of the moons radius plus the initial moon radius...when i get my final answer it is wrong...where did i go wrong for this problem?
  2. jcsd
  3. Mar 21, 2009 #2
    You have a discrete number of point masses, what's the moment of inertia of a point mass?
  4. Mar 21, 2009 #3
    what u mean? the moment of inertia for an individual rod?
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