Final answer:
The statement is true; normal stress acts on a surface that is perpendicular to the internal force of the member, consistent with the principles of normal force in mechanics.
Step-by-step explanation:
The statement that normal stress acts on a surface that is perpendicular to the internal force of the member is true. Normal stress, an engineering and physics concept, describes the stress component per unit area that arises from a force being applied perpendicular to the surface of a material. For example, when an object is resting on a horizontal surface, the normal force from the surface supports the object's weight and acts vertically upward, perpendicular to the surface. This is consistent with the normal force in mechanics, which holds regardless of the position of the member: it always acts perpendicular and away from the surface.