The discussion revolves around the sign convention for shear forces and bending moments in beam mechanics. Participants are questioning the author's choices regarding which directions are considered positive for shear forces and moments, particularly in the context of a beam subjected to various forces.
Participants do not appear to reach a consensus on the sign conventions used by the author, and multiple competing views regarding the reasoning behind these conventions remain present throughout the discussion.
Participants highlight the need for clarity on the assumptions underlying the author's sign conventions, particularly regarding the definitions of positive and negative directions for shear forces and moments.
chetzread said:
move on to the lower part of note, i found that the author take the clockwise moment at LHS as positive while at RHS , the author take anticlockwise moment as positive,why is it so?Clever Penguin said:If I had a beam, and applied a force acting upwards on the left, it would rotate clockwise.
If I was to instead apply a force acting downwards, but on the right, the beam would still rotate clockwise.
Sum of the clockwise moments = sum of the anti-clockwise moments. It is not about whether the force is acting up or down but whether the result of that force would cause it to rotate clockwise or anti-clockwise.
So, the moment clockwise considered as positive in this case?Clever Penguin said:If I had a beam, and applied a force acting upwards on the left, it would rotate clockwise.
If I was to instead apply a force acting downwards, but on the right, the beam would still rotate clockwise.
Sum of the clockwise moments = sum of the anti-clockwise moments. It is not about whether the force is acting up or down but whether the result of that force would cause it to rotate clockwise or anti-clockwise.