Question

Even when the rain is falling vertically downwards, the front screen of a moving car gets wet while the back screen remains dry. Why?

Answer 1

Hint
The rain possesses vertical motion downwards, while the car is also in a horizontal motion. When two bodies are simultaneously in motion, the effect of one on the other is explained by relative motion.

Complete step by step answer
We know that the velocity is a vector and thus, has both a magnitude and a direction. Consider a parked vehicle and rain falling on it. Since, the vehicle is stationary, whatever direction the rain falls in, the car will be hit by the raindrops from that direction only. To simplify, if the rain was falling vertically downwards, the rear and front end of the car will get wet at the same time and in the same manner, because the car is at rest with respect to the rain.
Now, when the car starts moving, it gets its own velocity component and direction in the horizontal way, while the rain is falling vertically downwards. This difference in directions would cause a resultant velocity direction which will be responsible for the droplets on the car. This velocity direction points towards the car from the front, and hence the front screen gets wet first, and then after some time the back screen.

Note
Suppose you are standing in the rain with a longitudinal wooden plank on your head. Assume rain falling vertical to you at constant speed. If you remain at the same place, you will notice the plank gets wet uniformly all over. But if you pick up a small speed in the rain, by the time the first droplet reaches the surface of the plank, you would have already moved forward. This would cause the rear end of the plank to still be hit by the rain, but the back end to remain dry. This is the principle behind the moving car and rain.