## Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (3rd Edition)

Let $v_w$ be the person's walking speed. Let $v_s$ be the speed of the moving sidewalk. Let $d$ be the distance traveled along the walkway. We can set up two equations of the form $v = \frac{d}{t}$: $v_w = \frac{d}{50~s}$ $v_s = \frac{d}{75~s}$ We can set up an equation for the combined speed: $v_w+v_s = \frac{d}{50~s}+\frac{d}{75~s}$ $v_w+v_s = \frac{3d}{150~s}+\frac{2d}{150~s}$ $v_w+v_s = \frac{5d}{150~s}$ $v_w+v_s = \frac{d}{30~s}$ It would take 30 seconds if the person walks while the sidewalk is moving.