Rain Streaks on a Moving Car: A Physical Explanation

Have you ever noticed raindrops hitting the side window of a car in motion and seemingly creating diagonal streaks even when there is no wind? It's a common and fascinating phenomenon that can be explained through the lens of physics. In this article, we will delve into the reasoning behind these diagonal streaks, breaking down the concepts of relative motion and vector summation.

Motion and Relative Velocity

When a car is in motion, it creates the illusion of wind coming from the front. Imagine you’re in a car traveling at 50 mph, and you extend your hand out the window. It feels as if you are experiencing a 50 mph wind from the front. This is because the car’s velocity is adding to the air currents around it, creating the sensation of strong wind.

Now, let's consider why the raindrops on the car's windows appear to create diagonal streaks. The relative motion between the car and the raindrops plays a crucial role here. The raindrops fall vertically due to gravity, but the car’s motion adds a horizontal component to their path, resulting in the observed diagonal streaks.

Vector Summation

To understand this phenomenon comprehensively, we can use the concept of vector summation. When the raindrops fall, they have a vertical velocity (due to gravity) and a horizontal velocity (due to the car’s motion). The resultant path of the raindrops is a vector sum of these two motions.

For example, if the car is moving at 11 m/s and the rain is falling at 10 m/s with a velocity vector pointing straight down, the angle made by the rain relative to the vertical can be calculated as follows:

Tan-1(11/10) 47.7°

This means the raindrops appear to be falling at an angle of 47.7° relative to the vertical. As a result, the raindrops create diagonal streaks on the car's windows.

Gravity and Airflow

Interestingly, the raindrops are not straight down; they are slanting in motion due to the Earth’s tilt. This tilt affects the direction in which rain falls, but the primary cause of the diagonal streaks is the car’s motion. Even under calm conditions, the rain appears to fall diagonally because of these factors combined.

Visualizing the Process

Imagine a raindrop falling vertically. As it falls, the car’s forward motion adds a horizontal component to its path. This combined motion results in the raindrop tracing a diagonal path on the car’s window, creating the illusion of diagonal streaks.

The diagram above illustrates how the raindrop’s vertical fall and the car’s horizontal motion result in a diagonal path on the window.

Conclusion

Next time you notice raindrops creating diagonal streaks on a car in motion, you’ll know the physics behind this fascinating phenomenon. The car’s motion adds a horizontal component to the vertical fall of rain, resulting in the observed diagonal streaks. Understanding this concept helps us appreciate the intricate interplay between motion, gravity, and optics in our everyday lives.