Sign convention of shear force and bending moment

[chetzread]

Homework Statement

http://nptel.ac.in/courses/112107146/22[/B]
in the notes, the author define upward shear force as positive (LHS) , he take downward shear force as positive as positive (RHS)?

Homework Equations

The Attempt at a Solution

why the author didnt fix downward shear force as positive , upward shear force as negative or vice versa

 

[Clever Penguin]

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.

why the author didnt fix downward shear force as positive , upward shear force as negative or vice versa

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.

 

[chetzread]

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.

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?
why the author take different direction of moment as same sign??

 

[chetzread]

why the author take clockwise moment as positive at LHS, take anticlockwise moment as positive at RHS?

 

[chetzread]

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?