# Which point to sum the moments?

1. Oct 24, 2012

### theBEAST

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. Oct 24, 2012

### haruspex

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.

3. Oct 24, 2012

### mishek

What about $ma_{x}$ and $ma_{y}$ in your moment equation?

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