support reaction

Jun 2014
306
0
I'm asked to find the W and I was told that the rope at C is in tension , there is reaction at B , my question is , is there any reaction at C ?


i have tried to do in this way , but i do not get the ans
vertical force = 80+10-RB+TC-RC-2W=0 --------equation 1
total moment about A = 80(1) +10(3)+W(2)(5) = 0
110+10W= 2RB , RB= (110+10W) / 2 ------------equation 2
total moment about B = -80(1)-2TC +2RC +10(1) +W(2)(3) =0

TC-RC = (6W-70) / 2 ----equation 3

Sub equation 2 and 3 into 1 ,
i gt 90-(110+10W) / 2 + (6W-70) / 2 -2W = 0
i gt 18W= 0
why cant i do int his way ?
 

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ChipB

PHF Helper
Jun 2010
2,367
292
Morristown, NJ USA
Yes there is a reaction at C. But first you need to decide what the body is that you are summing forces and moments for. If you consider just the horizontal bat then you should be including Tc forces (i.e the force applies at point A), but not Rc, as point C is not part of the bar. Or you may consider both the bar plus the element that connects A to C, in which case you include Rc but not Tc (since point A is interna; to the system). Do it one way or the other. If you consider just the horizontal bar you would have:

Sum of vertical forces = 0 = Tc +80 -Tb+ 10 +2W
Sum of moments about B = 0 = -2Tc-80(1)+10(1)+2W(3)

You have two equations and three unknowns, which means you can't solve for W unless you are given the value either of the reaction at point B or the tension in the element connecting A and C. I see you tried devising a second equation for sum of moments about point A, but that's redundant.
 
Last edited:
Jun 2014
306
0
Yes there is a reaction at C. But first you need to decide what the body is that you are summing forces and moments for. If you consider just the horizontal bat then you should be including Tc forces (i.e the force applies at point A), but not Rc, as point C is not part of the bar. Or you may consider both the bar plus the element that connects A to C, in which case you include Rc but not Tc (since point A is interna; to the system). Do it one way or the other. If you consider just the horizontal bar you would have:

Sum of vertical forces = 0 = Tc +80 -Tb+ 10 +2W
Sum of moments about B = 0 = -2Tc-80(1)+10(1)+2W(3)

You have two equations and three unknowns, which means you can't solve for W unless you are given the value either of the reaction at point B or the tension in the element connecting A and C. I see you tried devising a second equation for sum of moments about point A, but that's redundant.
why shouldnt we consider the RC in the calculation ??

Actually the original problem is what is the minimum W to be applied on the beam to make the cable in tension ? can i apply use the way u have done as above for this question ?
 
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ChipB

PHF Helper
Jun 2010
2,367
292
Morristown, NJ USA
why shouldnt we consider the RC in the calculation ??
See my earlier explanation - Point C is not part of the bar, so forces acting on point C are not part of the calculation of forces on the bar.

Actually the original problem is what is the minimum W to be applied on the beam to make the cable in tension ? can i apply use the way u have done as above for this question ?
Yes, just set Tc = 0. Then you can solve for Tb and W.
 
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