[philosoraptor.]

Ok, I'm looking at question 7 from this paper. (mark scheme.)

I do understand the answer given in the mark scheme but I can't figure out where my own method went wrong.
Resolving moments around C gives me:

w cos a = 3 R cos(a)
(w = 100N)

which gives R = 1/3 w - this is wrong but I can't see how the original equation is wrong.

Actually, the mark scheme also notes that resolving moments around X gives

2 R cos a = S (2 - 3 cos²a)

and I can't see where this comes from either. Surely as S is perpendicular to the rod it doesn't need to be multiplied by cos a (let alone cos²a)?

So if anyone can point out the (probably stupid) thing I'm missing that would be really helpful.

[philosoraptor.]

So in the two hours it took to get this thread approved, I actually ended up asking somewhere else. So it turns out I was ignoring friction, if anyone's interested.

w cos a + 3 f sin a = 3 R cos a

w cos a + 3 u R sin a = 3 R cos a

If you try to solve the problem this way, it quickly becomes clear that it's the wrong way to do it.