Problem 77 - Master Resource Book in JEE Main Physics

Question

A train is moving towards east and a car is along north, both with same speed. The observed direction of a car to the passenger in the train is (a) east-north direction (b) west-north direction (c) south-east direction (d) None of the above

Step-by-Step Solution

Answer

(b) west-north direction.

Step 1: Determine relative velocity of the car with respect to the train

Since both the train and the car are moving with the same speed but in different directions (one east and one north), we calculate the relative velocity of the car with respect to the train. The relative velocity is obtained by vector subtraction. Let's assume the speed is \( v \). Then the velocity of the car is \( v \hat{j} \) and the velocity of the train is \( v \hat{i} \). Therefore, the relative velocity of the car with respect to the train is \( v \hat{j} - v \hat{i} = -v \hat{i} + v \hat{j} \).

Step 2: Find the direction of the relative velocity vector

The relative velocity vector from step 1 is \( -v \hat{i} + v \hat{j} \). In terms of direction, this vector points towards the negative x-axis (west) and the positive y-axis (north). This means the car is moving in the west-north direction relative to the train.

Step 3: Match the direction with options

We have determined that the relative direction of the car as observed from the train is west-north. This corresponds to option (b) west-north direction.

Key Concepts

Vector Subtraction

When understanding relative velocity, vector subtraction plays a pivotal role. Let's imagine vectors represent the velocities of two objects, like a train and a car. Each vector shows both the speed and direction of its object.

In our example, the train moves towards the east with velocity vector \( v \hat{i} \), and the car travels north with velocity vector \( v \hat{j} \). To determine how fast the car appears to be moving from the train, we subtract the train's velocity vector from the car's velocity vector. This subtraction results in what we call the 'relative velocity'.

The calculation uses the following form: the car's velocity relative to the train is \( v \hat{j} - v \hat{i} = -v \hat{i} + v \hat{j} \). Think of it as comparing the difference in their speeds and directions. Vector subtraction isn't just about numbers; it visualizes direction in space, making it easier to understand how one moving object sees another.

Direction of Vectors

The direction of vectors is essential in interpreting the results of vector subtraction. A vector's direction is indicated by the combination of its components along the coordinate axes.

In the previous section, we found the car's relative velocity vector to be \(-v \hat{i} + v \hat{j}\). Here, the component \(-v \hat{i}\) represents motion westward, and \(v \hat{j}\) points northward. These directions align with the x and y axes conventions: the negative x-axis (west) and the positive y-axis (north).

Thus, to a passenger on the train, the car appears to travel diagonally in a northwest direction. Understanding vectors' directions allows us to visualize how objects appear to move when viewed from another moving object.

Velocity Components

Velocity components provide insight into the separate influences in two-dimensional motion. By breaking down velocity into components, we can analyze each direction individually.

For the train and car, both moving at velocity \( v \), their velocities relate to specific directions or 'components'. The train's velocity component is \( v \hat{i} \) heading east, while the car's is \( v \hat{j} \) going north. These components give us handy labels to describe complex motion in simpler terms.

This division into components allows easier manipulation in physics problems. For example, when calculating relative velocity, we handle each component as a separate part of the equation. This separation simplifies understanding how different axes contribute to the overall motion of an object. Essentially, velocity components help clarify motion, making it more intuitive to interpret results like direction and speed changes.