# Moments and trusses

1. Dec 15, 2013

Hi, i'm new on this website and it seems great :p So, I'm struggling with something really simple... trusses analysis. I'm struggling with moments etc , I've just drawn an example linked in this post , could you try to find the reaction forces (roller pin etc) with some explanation ? Thanks

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2. Dec 15, 2013

### tiny-tim

(please use the homework forum in future)

If I'm understanding the question correctly, no truss analysis is involved …

you have a rigid body with one applied force (plus its own weight), and you have to find the two reaction forces, so just use linear components of force, and moments …

show us what you get.

3. Dec 15, 2013

Thanks Tiny-tim for your response. In fact, I now for vertical external forces and horizontal ones so I divided the 100N vector in two vectors (one with a value of 50N orientated downwards and one with a value of 50•(square root of 3). I found that each pin has a vertical vector orientated upwards with a value of 25 N and for the fixed one, also an horizontal vector with a value of 50•(square roots of 3 orientated to the left). Am I right? I'm sure that I've made some mistake....what value did you find?

4. Dec 15, 2013

### tiny-tim

Correct.
Nooo …

you can't assume they're equal, can you?

all you know is that they add to 50N, so you need one more equation, which is … ?

5. Dec 15, 2013

The moment equation, right?
.... EMa=-50•3•12,1244 + 50(square root of 3)•7 + 4•12,1244•Rb ?

6. Dec 15, 2013

Where a is the pin fixed and b the roller pin

7. Dec 15, 2013

### tiny-tim

Yes.

(where 12.1244 = 14*√3/2)

Yes …

or you could avoid splitting the 100 N by doing 100 times the lever arm (the perpendicular distance from the line of the 100 N to the point A).

8. Dec 15, 2013

Yes but dont know how to do this because its not perpendicular to the line of action:)

9. Dec 15, 2013

Yes, but by doing my equation , I found that Rb=50N so R(vertical) is equal to 0??

10. Dec 15, 2013

### tiny-tim

use trig

11. Dec 15, 2013

yes but it's not working for me :p could you please try to explain why my equation is wrong ??? Please !! I'm getting a headache by trying and trying :p

12. Dec 15, 2013

### tiny-tim

I don't get 50

13. Dec 15, 2013

-50?

14. Dec 15, 2013

### tiny-tim

i mean, using your equation (which is a little confusing ) i don't get 50

15. Dec 15, 2013

Can you give me your result ? Is it 37,5?

16. Dec 15, 2013

### tiny-tim

17. Dec 15, 2013

No as you said before, we can't assume that both pin have the same vertical forces (25-25). Or, if Rby is equal to 25 and that EFy=0 <=> -50+25+Ray=0 :) so Ray is equal to 0 which is impossible

18. Dec 16, 2013

### tiny-tim

(just got up :zzz:)
yes, we can't assume it …

but that doesn't mean that we can't prove it (if it's true)
not following you …

doesn't -50+25+Ray=0 mean Ray = 25 ?