A 5-ft tall man is walking away from a street light 10 ft high at a speed of 5 ft/sec. How fast is the tip of his shadow moving along the ground?

Question

A 5-ft tall man is walking away from a street light 10 ft high at a speed of 5 ft/sec. How fast is the tip of his shadow moving along the ground? ​

Quizplus · Accepted Answer

The tip of the shadow moves at the rate of the man's speed plus the rate at which the shadow lengthens. Using similar triangles, the shadow lengthens at 5 ft/sec, so the tip of the shadow moves at 5 ft/sec + 5 ft/sec = 10 ft/sec.

A 5-Ft Tall Man Is Walking Away from a Street