# Finding reaction and moment

1. Dec 8, 2015

### goldfish9776

1. The problem statement, all variables and given/known data
Lets say i have a structure like this , at A and B are fixed support ( it resists moment about , force in x and y direction) I wanna find the moment about the A , should I include the MA as well in my calculation or no ?

2. Relevant equations

3. The attempt at a solution
moment about A = MB +MA + W(L/2) -RBy(L) = 0 ?
or moment about A = MB + W(L/2) -RBy(L) = 0 ?

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2. Dec 8, 2015

### PhanthomJay

Suppose this beam was a simple cantilever fixed at A but free at the other end. Same applied loading. Would you include the moment at A when summing moments about A in your moment equilibrium equation?

3. Dec 8, 2015

### goldfish9776

So, we don't have to consider MA when calculate the moment abour A?

4. Dec 8, 2015

### PhanthomJay

How did you come up with that conclusion?

5. Dec 8, 2015

### goldfish9776

Based on your tone

6. Dec 8, 2015

### PhanthomJay

Oh, bad tone. Try the simple cantilever case, say a beam A-B of length L fixed at A and with a load P applied at the free end B. Now sum moments about A = 0. Does MA show up in that equation or not?

7. Dec 8, 2015

### goldfish9776

no, how is it relate to the question i asked?

8. Dec 8, 2015

### PhanthomJay

In your problem, I cannot tell you whether or not to include MA in your equation when summing moments about A , because that is against forum rules . BTW, the problem is statically indeterminate. But anyway, as a hint, if you consider a simple cantilever of length L fixed at end A with the unknown moment M_A at that end and with a load P applied at the free end, then summing moments about A for equilibrium yields M_A - PL = 0. Thus M_A = PL as you would expect. The moment of any couple about any point is the couple itself. So back to your original problem about whether to include M_A in your equation...what say you now?

9. Dec 8, 2015

### goldfish9776

yes , i should include MA when i wanna calculate the moment about A

10. Dec 9, 2015

### PhanthomJay

yes, correct.

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