#### Problem 49E

49. A runner sprints around a circular track of radius $100 \mathrm{~m}$ at a constant speed of $7 \mathrm{~m} / \mathrm{s}$. The runner’s friend is standing at a distance $200 \mathrm{~m}$ from the center of the track. How fast is the distance between the friends changing when the distance between them is $200 \mathrm{~m}$ ?