Can someone help me with a simple statics calculation?

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

Been too long since I took Mech Eng classes but I know this would be cake for anyone in a first-term ME class:

What's the force in member A and B? Obviously compressive and tensile respectively. Please share the equation / method used!

BriceJohnson · **Joined:** Thu Apr 13, 2006 7:33 am **Posts:** 2230

Assuming small deflection (basically, that the long horizonal beam is rigid), you basically can make two equations, one that will show zero net force on the long cantilevered beam and one that shows zero net moment on the beam. Two variables, two equations, and when I solve (granted, before I've had my coffee) I get a force of 16.7 lbs on A and 26.7 on B.

Jason Tower · **Joined:** Wed Jun 11, 2008 3:31 pm **Posts:** 1173

i'm packing for road atlanta and my brain is never up to speed this early but three words come to mind: free body diagram (of the bar). it's static so:

sum of forces in x axis = 0
sum of forces in y axis = 0
sum of moments about any point = 0

no force in X so you get two equations with two vars, piece of cake.

Fa + Fb - 10 = 0 (y axis)
Fb = 10 - Fa

6Fa + 100 = 0 (moments about point b)
6Fa = -100
Fa = -16.7
Fb - 10 - (-16.7)
Fb = 26.7

i may have mixed up a +/- somewhere but you get the idea.

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

Sweet thanks guys, I had a FBD diagram drawn with forces but had a total mental block on balancling the moment

FYI this is an actual real world application - I am installing IR heaters on the ceiling in our screen porch, and I needed to create an offset drop bracket for it to move the heater 16" from the electrical box. I am trying to decide how many screws I should use to put it up and if I should consider adding a support chain.

So if I use 4 screws, the front ones will need to hold about 13lb each and the rears won't be seeing load at all really. Shouldn't be a problem unless the existing light plate twists.

Jason Tower · **Joined:** Wed Jun 11, 2008 3:31 pm **Posts:** 1173

cool, can't wait to see the final result! just remember the two rules that govern the universe: F=ma and OSU>UofM.

obviously i'm joking. F=ma only applies to classical mechanics, OSU>UofM is always valid.

JamesShort · **Posted:** Fri Feb 19, 2010 10:14 am

Brice, did you take statics with Gustafson? Man this brings back memories...I forgot a calculator the day of a test and he was just kind of a prick about it when I asked if I could solve everything analytically even if there is no calculations. I called my roommate and he hauled ass over to west campus with a calculator. I started the test 15 minutes late and was the only one in the class to finish all 3 problems. I got a 93 and the class average was below 60 even after he gave people extra time to finish the 3rd problem. I was pissed about that. Haha, anyway Dr. G was always cool other than day...must have rolled up on the wrong side of the bed that day.

BriceJohnson · **Joined:** Thu Apr 13, 2006 7:33 am **Posts:** 2230

I had Dr. Nadeau actually for statics, but I did have Dr G for a couple other classes, and he was pretty cool. I remember in one of his classes I questioned why I got a B+ instead of an A- b/c I thought based on the means and SD's on the tests I had done better. He broke out all the detailed calculations and his curve etc. and PROVED to me that I had gotten a curved 89.2. I wish all my other classes had been like that, instead of "class participation" being figured into the grade (otherwise known as "does the teacher like you"). I never questioned another grade from an engineering professor after that. . .

JamesShort · **Posted:** Fri Feb 19, 2010 10:36 am

BriceJohnson wrote:

I had Dr. Nadeau actually for statics, but I did have Dr G for a couple other classes, and he was pretty cool. I remember in one of his classes I questioned why I got a B+ instead of an A- b/c I thought based on the means and SD's on the tests I had done better. He broke out all the detailed calculations and his curve etc. and PROVED to me that I had gotten a curved 89.2. I wish all my other classes had been like that, instead of "class participation" being figured into the grade (otherwise known as "does the teacher like you"). I never questioned another grade from an engineering professor after that. . .

Yeah, I think my point was made when I walked out of the class on time and yet scored 4 SDs above the mean

. People just could not figure out the shear/moment diagrams for the life of them with non linear distributed loads.

Chris Brown · **Posted:** Fri Feb 19, 2010 10:39 am

did you guys get a lot of wedgies in middle school...just asking :wink:

David Spratte · **Posted:** Fri Feb 19, 2010 10:42 am

Chris Brown wrote:

did you guys get a lot of wedgies in middle school...just asking :wink:

The better question is if they kept track of them with an Excel sheet. . .

JamesShort · **Posted:** Fri Feb 19, 2010 10:47 am

David Spratte wrote:

Chris Brown wrote:

did you guys get a lot of wedgies in middle school...just asking :wink:

The better question is if they kept track of them with an Excel sheet. . .

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

Here are the drop brackets I made that I was modelling, and a shot of the ceiling where they are going up.

Think my crappy welds will hold 10lb?

Keith Quistorff · **Posted:** Fri Feb 19, 2010 11:50 am

Good to hear State has come around and have actual professors teaching Statics nowadays. Back in my day, I consistently had Middle Eastern grad students teaching my 200 level ME classes - it was hard enough to understand what they were saying, let alone the fact that they were completely green at teaching. I recall quite a few instances where they'd stumble through their lectures, making mistakes early in example problems and having to start over doing it the "right" way, which often ended up not being right either. I absolutely hated those classes, definitely a frustrating introduction to engineering.

JamesShort · **Posted:** Fri Feb 19, 2010 11:52 am

Keith Quistorff wrote:

Good to hear State has come around and have actual professors teaching Statics nowadays. Back in my day, I consistently had Middle Eastern grad students teaching my 200 level ME classes - it was hard enough to understand what they were saying, let alone the fact that they were completely green at teaching. I recall quite a few instances where they'd stumble through their lectures, making mistakes early in example problems and having to start over doing it the "right" way, which often ended up not being right either. I absolutely hated those classes, definitely a frustrating introduction to engineering.

Brice and I bleed a certain dark blue

not red.

When I TAed thermodynamics and heat transfer, the #1 comment on my end of course reviews was that english is my first language

.

Keith Quistorff · **Posted:** Fri Feb 19, 2010 12:14 pm

JamesShort wrote:

Brice and I bleed a certain dark blue

not red.

Oh, I'm sorry. :lol:

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Can someone help me with a simple statics calculation?

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