1. -More on Center of Gravity -More on Static Equilibrium
AP Physics C
Mrs. Coyle
Rio-Antirio Bridge, Coyle Summer 2009

2. Rio-Antirio Bridge, Greece Coyle, Summer 2009

3. Ex: Minnesota Metrodome Roof Collapse

4. Mechanical Equilibrium
Translational
Rotational
SF=0
Static (v=0)
Dynamic (v=Constant)
St=0
Static(ω=0)
Dynamic (ω=constant)

5. Necessary Condition for Equilibrium
The net (resultant) external force must be zero.
The net (resultant) extrenal torque must be zero about any axis.

6. Static Equilibrium SF=0 St=0 No rotation Static (v=0) Static(ω=0)
No translation
No rotation
SF=0
Static (v=0)
St=0
Static(ω=0)

7. Examples: What type of equilibrium exists?
A physics book sitting on a table.
A skater at constant velocity.
A spinning CD.
The wheel of a car traveling at constant velocity.

8. When objects are in equilibrium:
Σp=0 of its center of mass.
ΣL=0 about any axis.

9. Ex: Stable Equilibrium compared to Static Equilibrium
When an object returns to static equilibrium after it undergoes a displacement it is in stable equilibrium.
Ex: A steel ball at the bottom of a bowl when moved to the side of the bowl, returns to the bottom and eventually stops.

10. Remember: Center of Gravity
The point of the object where the force of gravity is thought to be acting.
Average location of weight.
If g is the same throughout the object then the CM coincides with the CG.

11. Problem Solving Strategy for Static Equilibrium
Draw the free body diagram of the object.
Set up the equations for: SF=0
Set up the equation for: St=0 (pick the axis with the simplest math).
Solve the equations for the unknowns.

12. Ex: 12.2 A person holds a 50N ball.
Find the upward force exerted by the bicep on the forearm and acting at the joint. Neglect the weight of the forearm.
Ans: 583N

13. Free Body Diagram

14. Ex: 12.3
A uniform beam is attached to a wall by a pin connection. A 600N person stands 2m from the wall. Find the tension in the cable and the magnitude and direction of the force exerted by the wall on the beam.
Ans: 313N, 580N , 71.10

15. Use the pivot at the wall as the axis.
The beam’s weight acts at its CG.

16. Analyze forces into x and y components

17. Ex: 12.4
A uniform ladder of mass m and length l, rests against a smooth vertical wall. If the coefficient of static friction between the ground and the ladder is 0.40, find the minimum angle θat which the ladder would not slip.
Ans:510