1. Licensed Electrical & Mechanical Engineer BMayer@ChabotCollege.edu
Engineering 36
Chp 5: Mech Equilibrium
Bruce Mayer, PE
Licensed Electrical & Mechanical Engineer

2. ReCall Equilibrium Conditions
Rigid Body in Static Equilibrium Characterized by
Balanced external forces and moments
Will impart no Tendency toward Translational or Rotational motion to the body
The NECESSARY and SUFFICIENT condition for the static equilibrium of a body are that the RESULTANT Force and Couple from all external forces form a system equivalent to zero

3. Equilibrium cont. Rigid Body Equilibrium Mathematically
Resolving into Rectangular Components the Resultant Forces & Moments Leads to an Equivalent Definition of Rigid Body Equilibrium

4. 2D Planar System In 2D systems it is assumed that
The System Geometry resides completely the XY Plane
There is NO Tendency to
Translate in the Z-Direction
Rotate about the X or Y Axes
These Conditions Simplify The Equilibrium Equations

5. 2D Planar System: No Z-Translation → NO Z-Directed Force:
No X or Y Rotation → NO X or Y Applied Moments

6. 2D Planar System:
If r Lies in the XY Plane, then rz = 0. With Fz = 0 the rxF Determinant:
So in this case M due to rxF is confined to the Z-Direction:

7. Example  Crane problem
Solution Plan
Create a free-body diagram for the crane
Determine Rcns at B by solving the equation for the sum of the moments of all forces about Pin-A
Note there will be no Moment contribution from the unknown reactions at A.
Determine the reactions at A by solving the equations for the sum of all horizontal force components and all vertical force components.
A fixed crane has a mass of kg and is used to lift a kg crate. It is held in place by a pin at A and a rocker at B. The center of gravity of the crane is located at G.
Determine the components of the reactions at A and B.

8. Example  Crane problem
Reaction Analysis
PIN at A
X & Y Reactions
ROCKER at B
NORMAL Reaction Only
+X Rcn in This Case
Solution Plan cont.
Check the values obtained for the reactions by verifying that the moments about B of all forces Sum to zero.

9. Example  Crane problem
Draw the Free Body Diagram (FBD)
Determine the reactions at Pt-A by solving the eqns for the sum of all horizontal & vertical forces
Determine Pt-B Rcn by solving the equation for the sum of the moments of all forces about Pt-A.

10. Example  Coal Car Solution Plan
Create a free-body diagram for the car with the coord system ALIGNED WITH the CABLE
Determine the reactions at the wheels by solving equations for the sum of moments about points above each axle, in the LoA of the Pull Cable
Determine the cable tension by solving the equation for the sum of force components parallel to the cable
A loaded coal car is at rest on an inclined track. The car has a gross weight of 5500 lb, for the car and its load as applied at G. The car is held in position by the cable.
Determine the TENSION in the cable and the REACTION at each pair of wheels.

11. Example  Coal Car Create Free Body Dia.
Determine the reactions at the wheels
Determine cable tension

12. Example  Cable Braced Beam
Solution Plan
Create a free-body diagram for the frame and cable.
Solve 3 equilibrium equations for the reaction force components and Moment at E.
Reaction Analysis
The Support at E is a CANTILEVER; can Resist
Planar Forces
Planar Moment
Also
The frame supports part of the roof of a small building. The tension in the cable is 150 kN.
Determine the reaction at the fixed end E.

13. Triangle Trig Review
SohCahToa

14. Example  Cable Braced Beam
Create a free-body diagram for the frame and cable
sin
cos
Forces are SLIDING VECTORS; they can be applied at ANY POINT on the Force LoA
Solve 3 equilibrium eqns for the reaction force components and couple

15. Rigid Body Equilibrium in 3D
SIX scalar equations are required to express the conditions for the equilibrium of a rigid body in the general THREE dimensional case
These equations can be solved for NO MORE than 6 unknowns
The Unknowns generally represent REACTIONS at Supports or Connections.

16. Rigid Body Equilibrium in 3D
The scalar equations are often conveniently obtained by applying the vector forms of the conditions for equilibrium
Solve the Above Eqns with the Usual Techniques; e.g., Determinant Operations
A Clever Choice for the Pivot Point, O, Can Eliminate from the Calculation as Many as 3 Unknowns (a PoC somewhere)

17. Example  3D Ball-n-Socket
Solution Plan
Create a free-body diagram of the Sign
Apply the conditions for static equilibrium to develop equations for the unknown reactions.
Reaction Analysis
Ball-n-Socket at A can Resist Translation in 3D
Can NOT Resist TWIST (moment) in Any Direction
Cables Pull on Their Geometric Axis
A sign of Uniform Density weighs 270 lb and is supported by a BALL-AND-SOCKET joint at A and by two Cables.
Determine the tension in each cable and the reaction at A.

18. 3D Ball-n-Socket Prob cont.
Create a free-body diagram of the Sign
The x-Axis rotation Means that the Sign is Only Partially Constrained. Mathematically
But If the Sign is NOT Swinging (i.e., it’s hanging STILL) The Following Condition Must Exist
LOAs for Tec & Tbc intersect the X-axis thus giving them a ZERO lever-arm relative to this axis. They thus impart no twist about the x-axis
With The Absence of the Mx Constraint This Problem Generates only 5 Unknowns
MX = 0 by X-Axis P.O.A. for W, TEC & TBC
Note: The Sign can SWING about the x-Axis (Wind Accommodation?)

19. 3D Ball-n-Socket Prob cont.2
State in Component-Form the Two Cable-Tension Vectors

20. 3D Ball-n-Socket Prob cont.3
Apply the conditions for static equilibrium to develop equations for the unknown reactions.
Solving 5 Eqns in 5 Unkwns

21. Solve 3D Problem by Mechanics & MATLAB 
>> F = [ ]
F =  
>> rxF = cross(r,F)
rxF =
WhiteBoard Work
Solve 3D Problem by Mechanics & MATLAB
ST * H 5-82 • see ENGR36_H13_Tutorial_3D_5DoF_HangingSign_H13e_5-82_1207.pptx
Determine the tensions in the cables and the components of reaction acting on the smooth collar at G necessary to hold the 2000 N sign in equilibrium. The sign weight may concentrated at the center of gravity.

25. Registered Electrical & Mechanical Engineer BMayer@ChabotCollege.edu
Engineering 36
Appendix
Bruce Mayer, PE
Registered Electrical & Mechanical Engineer