In an elevator moving upward at a constant speed, what will a spring scale read if it originally showed 18N?

When an elevator is moving upward at a constant speed, it is important to recognize that there is no net change in acceleration. According to Newton's first law, an object in motion will stay in motion at a constant velocity unless acted upon by a net force. Since the elevator is not accelerating, the forces acting on the scale remain balanced.

The spring scale measures the weight of an object based on the gravitational force acting on it. In this scenario, if the spring scale originally shows a reading of 18N while stationary, it continues to measure the same weight being applied to it even as it ascends at a constant speed. The gravitational force acting on the object does not change, nor does the scale's reading, because the upward motion does not contribute to any additional forces on the scale.

Thus, the reading of the spring scale will remain constant at 18N as the elevator moves upward, confirming that option C is the correct choice.