ch.3 equilibriumpioneer.chula.ac.th/~rchanat/2103213 mechi/ch3... · 2008-06-19 · r =∑f =0 r r...
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A body is in equilibrium, the resultant of all forces (force + couple) acting on it is zero.
0==∑FRrr
0==∑MMrr
3/2 System isolation and the free-body diagramBefore applying the equilibrium conditions, it must be cleared which body or system are analyzed, and which forces act on the body.
Free-Body Diagram (FBD)• The isolated system treated as a single body• All forces acting on body are shown
Ch.3 Equilibrium
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Modeling the action of forces(1)Type of contact Action on body to be isolated
1. Flexible cable, belt, chain or rope
Weight negligible
Weight not negligible
2. Smooth surfaces
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Modeling the action of forces(2)Type of contact Action on body to be isolated
3. Rough surfaces
4. Roller support
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Modeling the action of forces(3)Type of contact Action on body to be isolated
5. Freely sliding guide
6. Pin connection
Free to turn Not free
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Modeling the action of forces(4)Type of contact Action on body to be isolated
7. Built-in or fixed support
8. Gravitational attraction
9. Spring action
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Construction of FBD
1. Choose a body to be isolated
2. Draw “complete external boundary” of the isolated body
3. Add all forces and moments
4. Indicate a coordinate system
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Examples of FBD
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FBD problem(1)
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FBD problem(2)
Body Incomplete FBD
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FBD problem(3)
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FBD problem(4)
Body Wrong or Incomplete FBD
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FBD problem(5)
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FBD problem(6)