Running Dog

A man and a dog stand at opposite ends of a football field that is 100 feet long. Both start running towards each other.

Let’s say that the man runs at $x$ ft/s and the dog runs at twice the speed of the man. Each time the dog reaches the man, the dog runs back to the end of the field where it started and then back to the man and repeat.

What is the total distance the dog covers once the man finally reaches the end of the field?