What are the TWO conditions for a system to be in mechanical equilibrium?

What are similarities and differences between refraction, reflection, diffraction and absorption?

What determines the amount of inertia an object has

Determine the moment of the force f about an axis extending between a and c. express the result as a cartesian vector. a is at the origin, c is at (4,3,0) and f = (4i + 12j -3k) lb coming out of b (4,3,-2).