PHYS 218 sec Review Chap. 11 Equilibrium and Elasticity

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.)

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

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).

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 stable CG Not in equilibrium (non-vanishing net torque)

Ex 11.1 Range of CG for equilibrium

Ex 11.2

Ex 11.3 CG of the ladder

Ex 11.4 The negative sign means that E y 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.