Class 6 - Moment (Torque). Calculating Reaction Forces in a...

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Moment (Torque). Calculating Reaction Forces in a Truss. Class 06
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Protocol for Finding Force Components 1. Mark all angles CCW from the + x-axis 2. Then the components are ALWAYS given by  θ sin cos F F F F y x = = F θ F θ F θ θ F
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Sometimes it is not convenient  to use the suggested Protocol for  Finding Force Components. Then keep in mind: If force  F  vector points toward positive direction  of x-axis, then horizontal component of the  force is positive:   F x  > 0 If force  F  vector points toward negative direction  of x-axis, then horizontal component of the  force is negative:   F x  < 0 If force  F  vector points toward positive direction  of y-axis, then vertical component of the  force is positive:   F y  > 0 If force  F  vector points toward negative direction  of y-axis, then vertical component of the  force is negative:   F y  < 0 F F x y F F x y F F x y F F x y
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Force Equilibrium For Any Body to be in equilibrium: Algebraic Sum of all Forces must be Zero ! Same as the requirement that the Force polygon  must be closed. 0 0 0 x y z F F F = = =
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Recall Ex_05_06 Individually: Consider the simple seesaw, the  FBD is shown below. GIVEN: Ay=10 lb, AB=2 ft and  BC=3 ft, what should the value of  Cy be?  Solution: Principle of the lever:   the seesaw is in  static equilibrium , with  all forces balancing,  if AB*Ay = BC*Cy. 2*10=3*Cy Cy = 6.67 lb PIVOT A C B 2 ft       3 ft 10 lb
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Answer: Principle of the lever:   the seesaw is in  static equilibrium with all forces balancing,  if AB*Ay = BC*Cy. 2*10=3*Cy Cy = 6.67 lb PIVOT A C B 2 ft       3 ft 10 lb Ay Cy By
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Ex_05_07 Individually:  Now that we know Cy,  what is By? How did you find it? Sum of forces must be zero. So : -10 - 6.67 + By = 0 By = 16.67 lb PIVOT A C B 2 ft       3 ft 10 lb 10 lb                6.67 lb 2’       3’ Ay Cy By
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The story so far: Equilibrium What are the criteria for equilibrium?
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This note was uploaded on 04/27/2009 for the course ENGR 111 taught by Professor Walker during the Spring '07 term at Texas A&M.

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Class 6 - Moment (Torque). Calculating Reaction Forces in a...

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