The book remains at rest or, in physical terminology, in static equilibrium not because there are no forces acting on it but because the forces which do act on it annul one another.

• 此书处于静止，或者用物理术语来讲处于静力平衡，并不是因为没有力作用在它上面，而是因为作用其上的力彼此互相抵消。