FE Exam Statics - Force Members On A Truss Using Method Of Section

[@TKGxCOD]

This was a huge help!! quick question: For F-AB if that is compression because it is pulling towards B, then why isnt F-GB compression also since it is also pulling towards B? Thanks!

[@asmrmedley831]

For the last part with F(AB), when you take the summation of Fx, the positive direction is going towards the right. So when the answer ends up being negative 600 when it’s assumed to be in tension, that would mean it should be pointing towards joint A and be 600 in compression right?

[@aymanmuhsen099]

Engineers community, it was great oppurtunity to talk with Kenza yesterday, during my studying journey to do my exam I have thoroughly enjoyed the classes here, time of videos and the examples have been chosen ceafuly. I am highly recommand to review all videos and checking all other update practies exam subjects. Thank.

[@Genieprep]

Thank you Ayman, I really appreciate it and it was great talking to you as well 😊

[@drewnezelek6962]

I'm confused about the compression/tension, when you say "pulling away" from which joint exactly? Is it B because we took the moment there?

[@maidkahrimanovic1637]

@@drewnezelek6962 When she says "pulling away" she is talking about the joint at which the force is drawn. For the example that was used, at joint G, she drew force GH away from the joint and that answer came out positive, therefore it is a tension member.

[@kathleenmil402]

for Fgb why do we not include the 80kN force excerted at B? is it because it was not included in the cut? Why would that force not be included?

[@HearsH96]

Thank you so much! A little suggestion The way you describe the sign conventions for taking moments is a little confusing. Clockwise- negative Anti clockwise-positive would be much easier to interpret.

[@carultch]

You don't really need to care about the correct sign convention for a problem like this, and you will get the same answer if you assign your own signs. For this problem, all that matters is that clockwise moments add up to equal counter-clockwise moments, so that they both add up to zero. It is convention to assign a right-handed coordinate system, with x to the right, y up and z out of the page, and since moments and rotational vectors are assigned per the right hand rule, this means counter-clockwise will be positive, and clockwise will be negative. Mohr's circle is an example where you do need to care about following sign convention for shear stress, and there is an atypical case for it. Think "clock above, counter below". Some books draw a reversed axis for shear, other books simply assign the sign of shear stress the opposite way of rotational vectors in general.

[@congapilon]

in the moment at b eq. the 320N force at F has a 15 m perpendicular distance that was left out. correct?

[@jaymagnus154]

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[@ameerdylan3939]

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[@MunnukaYouTubeChannel]

When calculating the force for GH, considering moment at B=0, why are we not taking into account the negative value of Fx, i do understand that the negative is only for indicating whether it is tension or compression but when doing the summation of forces we do consider the negative sign so why not when we do it for moment summation

[@daniel083196]

Thank you. This really refreshed my mind

[@bolagerges8352]

AB FORCE Suppose to be -600KN

[@Genieprep]

That is right, compression has a negative sign and tension has a positive sign. So you can either say AB = 600 KN (C) or - 600 KN. In the FE exam, however, they would not give you multiple choice in terms of negative or positive but in terms of compression and tension. Good luck.

[@andreaprockup219]

@@Genieprep Hi, I thought if you got a negative sign it meant that you had made the wrong assumption. Like -600Kn we assumed to be in compression and i would have thought it would be in tension since it yield a negative result. Just like when we calculate the forces at the beginning that if it yields a negative then we made the wrong assumption for the direction. Could you please explain?

[@Genieprep]

@@andreaprockup219 Hello Andrea, I am not sure if I understand what you are saying. So with these problems, when we draw our FBD or our forces, we always draw them in tension, if we get a negative answer, it means that our assumption was wrong and the force is suppose to be in compression. I hope this helps and good luck with your studying!

[@andreaprockup219]

@@Genieprep Thank you for your help. I did not realize it was assumed to be in tension. It makes sense, thank you!

[@williamelder7011]

Assuming that Fab = -600 kn. Is it from joint "A" that we are determining whether tension or compression? I see that in the video you were looking from joint "B" perspective when determining the force.

[@nicolenoda2520]

Thank you so much for making this video I wanted to ask u why the force AB is in compression? I thought it was going away from the joint A so it would be in tension

[@Genieprep]

Nicole that's great question! First of all, when we cut, we draw all the forces in tension, and if we get a negative answer, that means the member is in compression. So if you look at Force AB, we have FAB [phone]+800=0, so FAB= -600, so negative means it is in compression! I hope this makes sense!

[@nicolenoda2520]

enGENIEer yes, thank you!! Great video btw!!☺️💕

[@JJYoko_Construction-Engineer]

Thanks for the refresher. One question, when is the force considered a zero or at which point that a force resolution considered zero?

[@hannahburawa1935]

When taking the moment at B, why was Ax=1,000 KN left out? Shouldn't the sum of the moment at B = -Fgh(12) + 1200(12) - 320(15) + 1000(12) = 0?

[@hannahburawa1935]

Oh, wait! Do we not include Ax because it is acting through the moment at B? Similar to how Fy is not considered when we take the moment at A.

[@koashaygyi]

Thank you

[@quynhcao5801]

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[@annepatsy1060]

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