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Homework Help: Which point to sum the moments?

  1. Oct 24, 2012 #1
    1. The problem statement, all variables and given/known data
    Here is the problem:

    Here is the solution:

    I am confused about part c.

    3. The attempt at a solution
    Here is my attempt at part c:

    I don't get why the answer key solution sums the moments about point G. I decided to sum up the moments about C and D and found out both normal forces are 0.5mg. However I quickly find out this is wrong because I am unable to solve the rest of the problem (which I did not include).
  2. jcsd
  3. Oct 24, 2012 #2


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    Gold Member

    If you take moments about D, say, you should not get a zero result. The crate's mass centre is accelerating to maintain its centripetal motion. This requires a moment about D. Taking moments about G avoids this.
  4. Oct 24, 2012 #3
    What about [itex]ma_{x}[/itex] and [itex]ma_{y}[/itex] in your moment equation?

    P.S. Sorry haruspex, I didn't see you already replied.
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