# Why can contact force be ignored here?

I was discussing the first part of this question with a friend, and he told me that while taking moments about $A$ and $B$, we can ignore the reaction force at C. Indeed this is correct, because ignoring the reaction force at C gives the right answer. However, what I don't understand is that why can we do this?

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• I saw classical mechanics as I tag. Isn't this included in that? – user140161 Feb 9 '15 at 19:46
• Also, I included that this applies while taking moments about A and B. How does the line of action of the reaction at C pass through the A or B? – user140161 Feb 9 '15 at 19:48
• Then why would we take the reaction for at B while calculating moments about A, or the reaction force at A, while taking moments about B? – user140161 Feb 9 '15 at 20:02
• You should know when you are talking about a system and when you are talking about an isolated object. – Troy Woo Feb 9 '15 at 20:11