Homework Help: Statics equilibrium question

1. Aug 5, 2012

Kasthuri

1. The problem statement, all variables and given/known data

Question: The plate shown, picture is attached, is subjected to force F and 3 reaction forces R1 R2 R3. Find R1

2. Relevant equations

equations of equilibrium:
sum of Fx = 0
sum of Fy = 0
sum of moment about point O = 0

3. The attempt at a solution

sum of Fx = 0: (going right is positive)

-R1 - R2 + 3/5 R3 = 0

sum of Fy = 0: (going up is positive)

-56kN + 4/5 R3 = 0
so, R3 = 70kN

sum of moment about point O = 0 (going anticlockwise is positive)

R1 x (0.11m) - R2 x (0.11m) = 0
so, R1 = R2

therefore, R1 = R2 = 21kN

I am unsure of whether I have done this question correctly, in particular the moment about O.
Also, how do you figure out which direction the moment is in (anticlockwise/clockwise). As in, I took R1 x 0.11m as positive due to anticlockwise and
R2 x 0.11m as negative due to clockwise moment.
is this correct?

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2. Aug 5, 2012

CWatters

R3=70Kn appears correct.

As for the moment...

R1*110 + (-R2*110) = 0

So

R1 = R2

Both same sign

Horizontally, if left is positive...

-R3*3/5 + R1 + R2 = 0

-R3*3/5 + R1 + R1 = 0

-R3*3/5 + 2R1 = 0

R1 = R3*3/(5*2)

= 21kN

which is a +ve number so to the left.

R1 = R2

so R2 also to the left.

3. Aug 5, 2012

Thank you!!

4. Aug 6, 2012

pongo38

To check whether you are correct, just take moments about any other point. The sum should be zero.