A wind is blowing straight from north to south at an unknown speed. When you hold up a piece of thread you can see that the wind stretches it out so that it points from your hand to the south. Now you get on your bicycle and ride due east and note that your metric speedometer reads 7.0m/s. When you look at the thread you are still holding up you see that it now points from your hand in the direction 30° west of south. What is the speed of the wind relative to the ground?