I have a conceptual doubt concerning the last question. So for (a) the resultant moment is 120 N anti-clockwise since the forces are acting in opposite directions, $f_1 \cdot d_1 = f_2 \cdot d_2$ thus:
$$ 600-480= 120 {\rm Nm} $$
For (b) the resultant moment is the added moments of both forces since they r acting in the same direction this:
$$ 600+480 = 1080 {\rm Nm} $$
The answer to b(ii) is conceptually confusing. I understand why the resultant force is 100 N to some extent, especially if I use the logic of contact force. As far as I know the contact force at the pivot opposes the other forces. So if 500 N is being exerted by mechanic A downwards contact force is 500 N upwards at the pivot.
And if 400 N is being exerted by mechanic B upwards then 400 N is being exerted downwards from the pivot.
Therefore 500 N upwards- 400 N downwards gives me 100 N p force at the pivot upwards. That means a resultant force of 100 N acts downwards which we can intuitively say acts from the left end of the spanner since the moment arises from that direction.
The mark scheme for Cambridge igcse physics says the answer is 100 N. But my question is if the resultant force is 100 N that would mean the resultant moment is 120 N m which would be incorrect since we just figured out that the resultant moment is 1080 N (which the mark scheme says is correct).
So what concept am I missing here, it doesn’t make much sense to me.