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Method of sections

  • #1

Homework Statement



Find the force in each member of an equilateral truss. See figure.

Homework Equations



sum Fy=0
sum Fx=0
sum M=0

The Attempt at a Solution



first of all i tryed to cut through AB and AC and i off set the X/Y axis by rotating 30 degrees CCW then i did,

0=Fbc-735cos60

i thought i would get the right answer for Fbc but it is wrong.
Any thoughts?
 

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Answers and Replies

  • #2
741
27
First of all, try always to draw to scale, in order to avoid distorting the problem. Secondly, when you make a cut, as you have done, replace the cut ends with the forces that are "exposed", in direction if you don't know the magnitude. Then make equilibrium statements about the free body diagram that results, with all the horizontal and vertical components if necessary. In this case, are you able to sketch the triangle of forces at B, more or less to scale? The equation you attempted ignored the contribution from BC.
 
  • #3
741
27
Sorry, that last sentence should have said "The equation you attempted ignored the contribution from AB.
 

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