Cats n Dogs PROBLEM

Question. A sitting cat in a field suddenly saw a dangerous bull dog at a distance l from her. To save her precious life, the cat runs away in a straight line with const. velocity u. Without any time lag the dog starts running with a const. speed v (v>u) to catch the cat. At initial moment v is perp. to u. If the dog always heads towards ths cat, find the time after which the dog catches the cat?????

Show ur steps also.