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PHYS 218 sec. 517-520

This review covers essential concepts of equilibrium and elasticity as detailed in sections 11.1 to 11.3. Topics include conditions for equilibrium of rigid bodies, methods for solving equilibrium problems, and the significance of the center of gravity (CG). We explore both the first and second conditions for equilibrium, emphasizing the importance of gravity's role in creating rotational forces. Practical examples illustrate calculations involving CG and torques, ensuring a solid grasp of how forces interact to maintain stability.

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PHYS 218 sec. 517-520

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  1. PHYS 218sec. 517-520 Review Chap. 11 Equilibrium and Elasticity

  2. What you have to know • Conditions for equilibrium for a rigid-body • How to solve rigid-body equilibrium problems • Only Sections 11.1 – 11.3 are reviewed. (We skipped the other sections in the class.)

  3. Conditions for equilibrium First condition for equilibrium Second condition for equilibrium If the body is at rest, it is in static equilibrium.

  4. Center of Gravity In most equilibrium problems, gravity is one of the forces that make the body rotate. We can assume that the entire force of gravity is concentrated at the center of gravity (CG) of the body. If the change in the gravitational acceleration g can be ignored, the center of gravity (CG) of the body is identical to its center of mass (CM).

  5. Center of Gravity The total gravitational torque is the same as though the total weight were acting on the CM of the body. A body supported at several points must have its CG somewhere within the area bounded by the supports. CG CG stable Not in equilibrium (non-vanishing net torque)

  6. Ex 11.1 Range of CG for equilibrium

  7. Ex 11.2

  8. Ex 11.3 CG of the ladder

  9. Ex 11.4 The negative sign means that Ey is downward. If you don’t know the direction of a force a priori, then choose an arbitrary direction. After calculation, if you get a negative value, then it means that the direction of the force is opposite to your guess.

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