# EQUILIBRIUM AND ELASTICITY

I. What is equilibrium?

A. The object remains at rest or moves in a straight line at constant speed.

B. The object is not rotating or is rotating at a constant angular velocity.

II. If there is no rotation there are restrictions on the force vectors for cases of equilibrium. Generally, the vector sum of the forces must be zero. However,

A. If only two forces act they must have the same line of action.

B. If three or more forces act on an object they must be concurrent.

III. The second condition for equilibrium is that the vector sum of the torques is zero.

IV. Center of gravity - The gravitational forces acting on individual mass elements that make up a rigid body are equivalent in their translational and rotational effects to a single force W acting at the center of gravity.

EXAMPLE

Find the tension in the horizontal wire.

ELASTICITY

I. All "rigid" objects are to some extent elastic.

A. Stress - deforming force per unit area

B. Strain - a unit deformation

C. A modification of Hooke's Law (F = -kx): stress = modulus x strain

II. What is tensile stress?

III. What is shear stress?

IV. What is "hydraulic stress"?