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Homework Help: Simple problem involving moment and force

  1. Sep 25, 2010 #1
    1. The problem statement, all variables and given/known data

    The two systems are equivalent. If F = 30i - 100j + 40k lb and M' = 40i - 80j + 120k lb*in, determine F' and M.

    Here's a pic of the problem: http://img214.imageshack.us/img214/339/fc1g.jpg [Broken]

    2. Relevant equations

    M = r x F

    3. The attempt at a solution

    To find M, I started out finding the position vector for System 1 and got r = 6i + 0j +6k, and then did the cross product of that with F = 3i - 100j + 40k, and got M = 600i - 222j - 600k. To find F' for System 2, I took r to be 6i + 4j + 6k. I then set up the cross product box, with the F' row blank, and tried finding F' with the known answer of M' = 40i - 80j + 120k, however it is practically impossible to solve for F' that way and something tells me that's not the right way to try to solve it anyway. Also, something tells me that I didn't solve for M in the first part correctly either.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Sep 25, 2010 #2


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    RKOwens4: Hint 1: Ensure you copied the numbers given in the problem statement correctly in your calculations. Hint 2: If F' is not identical to F, would the system be subjected to different forces?
  4. Sep 25, 2010 #3
    Sorry but I'm really not getting anything helpful from that. Of course it would be subjected to different forces, I just don't know how to calculate F' or M.
  5. Sep 26, 2010 #4
    Come on, guys. EIGHTY-FIVE views and not one person can help me? EIGHTY-FIVE?
  6. Sep 27, 2010 #5
    RKOwens4: What do you think it means to say "the two systems are equivalent"? Have you used that property?
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