Beam Deflection Homework: Missing Moment Calculation for Beam Circled Section

[chetzread]

Homework Statement

for the circled part, why the why the author didnt include -12(9)kNm moment in the equation of moment ?
[PLAIN]http://imgur.com/a/l3Bu4[URL]http://imgur.com/a/l3Bu4[/URL]

Homework Equations

The Attempt at a Solution

IMO, the author should include it in the equation, am i right?
So, M= -258+52<x-0>-(0.5)(8)(<x-0>^2)+50+(0.5)(8)(<x-5>^2)-12(9) ?

 

[chetzread]

http://imgur.com/a/l3Bu4

 

[chetzread]

did the author miss out something?

 

[pongo38]

The figure does not include the variable 'x'. But, wherever you put it in the range 0 to 9 m, what contribution to Mx do you think the 12 kN force would make? Recall one definition of moment that it is the algebraic sum of moments on ONE side of a section.

 

[chetzread]

The figure does not include the variable 'x'. But, wherever you put it in the range 0 to 9 m, what contribution to Mx do you think the 12 kN force would make? Recall one definition of moment that it is the algebraic sum of moments on ONE side of a
section.

The force 12kN only will cause moment at x just 9m, no moment generated at x less than 9m...
But, I still couldn't understand why the moment 12(9) is not included?

 

[chetzread]

The figure does not include the variable 'x'. But, wherever you put it in the range 0 to 9 m, what contribution to Mx do you think the 12 kN force would make? Recall one definition of moment that it is the algebraic sum of moments on ONE side of a section.

do you mean the x value range from 0 to 9 ? it can't be more than 9 , because there's no moment beyond 9m ?

 

[pongo38]

Because the expression for M is the summation of moments to the LEFT of section X, but the 12 kN load is to the right. If you were to write down an expression for the moment to the RIGHT of x, you would get an expression that may at first sight look different, but, if correctly drawn up, will be exactly equivalent (and incidentally acts as a check on whether you have made any mistakes of the kind you are making now). I suggest you try that to reinforce the correct application of the definition of bending moment.