Читать книгу Flight Theory and Aerodynamics - Joseph R. Badick - Страница 25

Webster defines equilibrium as “a state in which opposing forces or actions are balanced so that one is not stronger or greater than the other.” A body must meet two requirements to be in a state of equilibrium:

1 There must be no unbalanced forces acting on the body. This is written as the mathematical formula ∑F = 0, where ∑ (cap sigma) is the Greek symbol for “sum of.” Figure 1.2 illustrates the situation where this condition is satisfied (lift = weight, thrust = drag, etc.)

2 There must be no unbalanced moments acting on the body. Mathematically, ∑M = 0.Moments at the fulcrum in Fig. 1.8 are 5000 ft‐lb clockwise and 5000 ft‐lb counterclockwise. The weight (force) of A is 100 lb and is located 50 inches (″) to the left of datum (fulcrum), thus 100 lb × −50″ = −5000 lb‐in. The weight of B is 200 lb and is located 25 inches to the right of datum, thus 200 lb × 25″ = 5000 lb‐in. So, ∑M = 0.