Final answer:
Normal stress in a bar subjected to axial force depends on the cross-sectional area, and without specific dimensions, it cannot be definitively stated whether replacing a rectangular bar with a circular or square bar will result in higher or lower normal stress.
Step-by-step explanation:
When a rectangular bar is subjected to an axial force and then replaced by a bar of another shape, the normal stress in the new bar will depend on its cross-sectional area relative to the original bar. Normal stress is defined as the force per unit area (stress = force / area), and for a given axial force, if the area is smaller, the stress is higher and vice versa.
In the case of changing from a rectangular bar to a circular bar, without additional information about the dimensions, it's not possible to definitively say if the normal stress will be higher or lower. If the cross-sectional area of the circular bar is less than that of the rectangular bar, the stress would be higher. Conversely, if it is larger, the stress would be lower. The same logic applies to replacing the original bar with a square bar.