How to solve 3D statics problems 

Glad I could help! Check out all the statics vids I did at engineer4free.com/statics if you haven't already!! ✌️

Glad I could help! Check out all the statics vids I did at engineer4free.com/statics if you haven't already!! ✌️

The general method is described in the last half of this video, and the two following videos in the playlist use this method and go through the calculation of moments about the x, y, and z axis. Links are here: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-sEJmLnrnY28.html and here: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-7zist6Y6N8g.html

Engineer4Free could you do some videos on dynamic general plane motion of rigid bodies, and do a couple of examples of these sort of topics as I am finding them hard.

Awesome, thanks for letting me know!

Yes you would have to also consider the moments cause by the reaction forces at A in the situation that you described. It would be possible to do with the same formula M = r X F where F is each reaction force and r is the position vector with tail at the point in question and head at the reaction in each case, lets call it rq in the next step. The expression for some arbitrary point "q" for this same example with the three applied point loads would be ΣMq = May + Maz + Max + r1XF1 + r2XF2 + r3XF3+ rqXFx + rqXFy + rqXFz = 0. The reason the last 3 terms don't show up in the expression for the sum of moments about A in the video is because their lines of action all pass through A, which means they do not cause a moment about A. In order to find the moment about another point q, you would need to know the fist 6 terms in the equation which you get by solving for the reaction at A anyways. I think you might be overthinking it if you are in a statics course right now. Statics problem should always just be focusing on trying to find the unknown reactions of statically determinate structures like in this video. The reason someone might want to find the net moment about some point that is not a reaction would be to determine the internal forces/moments/stresses at that location, but typically the reasons for wanting that information have to do with some topics in the field of mechanics of materials or structural analysis/design. So again, yes you are correct in your thought to also include the moments caused by the reaction forces, but I just think there's a pretty good chance that you don't need to be going down that road just yet if you're studying statics. I hope that clears it up and doesn't confuse you more haha :)

Glad to hear it =)

It's often to eyeball moments in 3D problems. 2D problems are much easier. I recommend checking out the "Moments" and "Three-Dimensional Static Equilibrium" sections here: engineer4free.com/statics

@@Engineer4Free thanks am going on to watch your lecture

You're welcome!

Thanks Jacqueline!! 🙌🙌

You're welcome!!

i'm also interested in computer science stuff. i used to want to be a surgeon but i heard it's hard

Those are existential questions that I can't answer for you. I'll keep my explanations limited to solving 3D statics problems 🙃

Yes an (incorrect )negative answer is to be expected if you put the order backwards. You just need to remember to put the order correctly. Please see videos 9 - 12 here: engineer4free.com/statics for statics tutorials about the vector cross product

@@Engineer4Free thank you. i will watch ASAP. but please for quick question: everytime i do 2D statics problem, i never use vector. so yea i always do M = F times r (yes, scalar) . i forgot that it is actually r cross F. turns out i got negative value. will i still positive value at the final result? i mean, all of these equations equal to zero. so i think it doesn't matter if i start it with negative value, as long as i consistently (wrongly) do F x r? will it result in the same value as the correct r x F?

Ahh sorry I only have videos on how to solve this way, once you get the hang of it I think it's the easiest / fastest way to solve 3D statics problems. Check out videos 38-41 here engineer4free.com/statics the more practice the easier it gets!