Mechanical work

When a force acts on a body and moves it, the force does mechanical work. If the force is along the displacement, the work equals the product of the magnitudes: A = Fs. If the force is at an angle to the displacement, only its projection on the direction of displacement does work: A = F_s · s. The SI unit of work is the joule (J): 1 J = 1 N · 1 m, the work done by a 1 N force moving a body 1 m along the force. For large amounts, kilojoules: 1 kJ = 1000 J. If there is no displacement the work is zero — pushing a wall until tired does no work in the physical sense. If the force is perpendicular to the displacement (its projection is zero) the work is zero; if the force is opposite to the displacement (for example friction) the work is negative. Lifting a body uniformly by h: A = mgh.