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ENGR210, Fall 2011
1
Problem solving processes for a system in static equilibrium
1. Draw FBDs
11. Identify a coordinate system
12. Draw outlines of the system
Write known quantities (magnitude, direction, etc.)
Write unknowns (force, length, angle, etc.)
2. Write static equilibrium equations
For a 2D particle system,
∑ ܨݔ ൌ 0, ∑ ܨݕ ൌ 0
(2 eqns)
For a 3D particle system,
∑ ܨݔ ൌ 0, ∑ ܨݕ ൌ 0, ∑ ܨݖ ൌ 0
(3 eqns)
3. Identify additional equations, if necessary
(e.g., spring force (Fs=kl),
the Pythagorean theorem, etc.)
* The number of unknowns you write in your FBDs has to be the same
as the number of equations in 2 and 3 to solve for the unknowns.
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2
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ENGR210, Fall 2011
3
Comments:
The solution above started with the node (particle) A, and
therefore, could immediately solve for T
AB
and T
AC
(i.e., two
equilibrium equations and two unknowns). If you start with
the node B, you will not be able to solve for the three
unknown tension forces (T
AB
,T
BC
and T
BE
) because there are
only two equilibrium equations available at B. However, by
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This note was uploaded on 10/16/2011 for the course ENGR 210 taught by Professor Ko during the Fall '08 term at Boise State.
 Fall '08
 KO
 Statics

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