Question

A car is moving on a road and rain is falling vertically. Select the correct option.
A. The rain will strike the back screen only
B. The rain will strike the front screen only
C. The rain will strike both the screens
D. The rain will not strike any of the screens

Answer 1

Hint : To solve the given question, use the concept of relative motion. To make things easy and understandable assume a specific direction for the velocity of the car and draw a vector diagram of the velocities of the car and the rain. Then find the direction of the rain with respect to the car.

Complete Step By Step Answer:
It is given that a car is moving on a road and rain is falling down vertically.
Now, since the car is in motion, the man inside the car will see the rain coming in a different direction. The direction of the rain will be with the motion of the man, i.e. with respect to the motion of the car.
To find the direction of the rainfall with respect to the car, we have to apply the concept of relative motion. According to the concept of relative motion (like velocity and acceleration), the motion of body B with respect to a body A is equal to difference in the vectors of motion of B and A.
i.e. $\overrightarrow{{{v}_{BA}}}=\overrightarrow{{{v}_{B}}}-\overrightarrow{{{v}_{A}}}$ , where $\overrightarrow{{{v}_{BA}}}$ is relative motion of B with respect to A.
Suppose the car is moving along the positive x-axis and the rain is falling down along the negative y-axis.
Let the velocity of the car be $\overrightarrow{{{v}_{A}}}$ and the velocity of the rain be $\overrightarrow{{{v}_{B}}}$

Then, the velocity of rain with respect to car is $\overrightarrow{{{v}_{BA}}}=\overrightarrow{{{v}_{B}}}-\overrightarrow{{{v}_{A}}}$
From the figure we understand that $\overrightarrow{{{v}_{BA}}}$ is somewhere towards the south-west and the car is moving towards the east. Therefore, the rain will hit the front part of the car.
Hence, the correct option is B.

Note :
Note that the concept of relative motion is only applicable for physical quantities that are vectors. For example, velocities, acceleration and momentum.
Also note that we can calculate the relative motion between two bodies for the same physical quantities. For example, we cannot calculate relative acceleration of a body with respect to the velocity of another.