## statics- beams internal effects...i dont get it

1. The problem statement, all variables and given/known data
this is a picture of an example from the book. im going off of it to solve a problem that will be on my test tomorrow. i can figure out how to calculate the shear force and moment of each section (though i dont fully understand it), but im just not getting how they decide on these sections. sorry for the crappy photo, all i have it my camera phone.

the sections are labeled like 0<x<4, 4<x<8 etc. but when they draw the 4<x<8 section in the example they include the 0<x<4 section....? why? and how come x changes with every section. thats confusing the hell outta me. oh and on the first section diagram to the left, how did they come up with (x/4)*100? and 12.5x^2?

thanx

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
 PhysOrg.com science news on PhysOrg.com >> New language discovery reveals linguistic insights>> US official: Solar plane to help ground energy use (Update)>> Four microphones, computer algorithm enough to produce 3-D model of simple, convex room
 thanx, i understand that part, its just the whole x thing that confuses me. i know im not explaining myself very well, sorry. my dad just explained to me where 100x/4 and 12.5x^2 come from so i got that part now. you know what, screw it. i think i know it enough to be able to solve the problem. i think i just need help with the final part of calculating the max moment and its location x from the left. my dad pointed out that if you take the derivative of the moment from the 4

Recognitions:
Gold Member
Homework Help

## statics- beams internal effects...i dont get it

 Quote by Fat Ryan thanx, i understand that part, its just the whole x thing that confuses me. i know im not explaining myself very well, sorry. my dad just explained to me where 100x/4 and 12.5x^2 come from so i got that part now. you know what, screw it. i think i know it enough to be able to solve the problem. i think i just need help with the final part of calculating the max moment and its location x from the left. my dad pointed out that if you take the derivative of the moment from the 4
Points of maximum moments occur at points where there is no shear (i.e., where dM/dx = V = 0, as your dad pointed out). I general, there may be more than one location where the moments are at a 'maximum' in a particular section, so it is wise to check all sections where the shear in that section is zero (draw a good shear diagram, and note where the shear crosses the x axis (V=0), then draw a moment diagram). A 'maximum' moment in one section may be greater than a 'maximum' moment in another.
 ok thanx. well i def failed the test today. i dont think i did too bad on the beam problem, but i did HORRIBLE on the centroid problem :( i think my prof picked the hardest possible centroid problem and then made it even harder.

Recognitions:
Gold Member
Homework Help