Raindrops hitting the side windows of a car in motion often leave diagonal streaks even if there is no wind. Why? Is the explanation the same or different for diagonal streaks on the windshield?
Solution to 13DQ Consider the following figure, Step 1 We will assume that there is no wind. When the car is stationary, there is no airflow around the car. Thus the rain drops vertically hitting the car will show vertical streaks on both windshield and the window. Now consider the car is moving forward with a velocity v. There will be a flow of air across the car in opposite direction. This airflow will carry the raindrops along its direction. But the gravitational force acting on the raindrops will try to oppose this change, the resultant of action of airflow and gravity would carry the rain diagonally across the window. Thus the airflow across the moving car is the reason why rain drops leaves diagonal streaks in the window and windshield of a moving car.
