The normal force on a wood block equals zero when there is no force pressing down on it, such as in a state of free fall or when the block is in a weightless environment, like space. This occurs because the normal force acts perpendicular to a surface to counteract gravity, and if gravity is negligible, the normal force disappears. Additionally, when a block is on a vertical ramp, the normal force can also be zero due to the lack of contact force acting perpendicular to the surface. In equilibrium, one object must exert a force on another to maintain balance, but without any external forces, the normal force cannot exist. Understanding these conditions is crucial for calculating normal force in various scenarios.