Chapter 9 Bodies in Equilibrium

What constitutes equilibrium?

We have discussed static equilibrium for the case of point objects. We required that the sum of the forces in the X direction = 0, and the sum of the forces in the Y direction = 0. For an extended object we must also require that the sum of the torques = 0.

First Condition for Equilibrium

Second Condition for Equilibrium

1. TRUE or FALSE If the net force on an object is non-zero the torque on the object will be non-zero.

2. TRUE or FALSE If there are two forces acting on an object such that the forces have equal magnitude but opposite directions, the object will be in a state of static equilibrium.

3. TRUE or FALSE The center of mass of an object is always someplace inside the physical body of the object.

Make a good free-body diagram. Only show forces that act on the object.

Choose an X-Y coordinate system that is convenient.

Choose an axis of rotation location that is convenient.

Write down the necessary equations of static equilibrium.

4. Is a joint a reasonable location for an axis of rotation?

5. What must be true for an object to be in stable equilibrium?

6. What is true when an object is in unstable equilibrium?

We will skip sections 6-8.

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