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JEE Advanced 2008 · Motion in a Straight Line

STATEMENT-1 : For an observer looking out through the window of a fast moving train, the nearby objects appear to move in the opposite direction to the train, while the distant objects appear to be stationary. STATEMENT-2 : If the observer and the object are moving at velocities v1v_1 and v2v_2 respectively with reference to a laboratory frame, the velocity of the object with respect to the observer is v2−v1v_2 - v_1.

Answer: B. Statement-1 is True, Statement-2 is True; Statement-2 is NOT a correct explanation for Statement-1

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Written solution

Step 1: Analyze Statement-2.

The relative velocity of an object with respect to an observer is defined by the vector difference of their velocities in a common reference frame.

v⃗rel=v⃗2−v⃗1\vec{v}_{\text{rel}} = \vec{v}_2 - \vec{v}_1

This is a standard definition and is always true. Thus, Statement-2 is true.

Step 2: Analyze Statement-1.

For stationary objects on the ground, their velocity in the laboratory frame is v⃗2=0\vec{v}_2 = 0, meaning their relative velocity with respect to the train is constant.

v⃗rel=−v⃗1\vec{v}_{\text{rel}} = -\vec{v}_1

The apparent motion perceived by the observer is determined by the angular velocity ω\omega, which is the rate of change of the line-of-sight angle θ\theta.

ω=dθdt≈v1D\omega = \frac{d\theta}{dt} \approx \frac{v_1}{D}

For nearby objects, the distance DD is small, resulting in a large angular velocity ω\omega, so they appear to move rapidly backward.

For distant objects, the distance DD is very large, making ω→0\omega \to 0, so they appear stationary. Thus, Statement-1 is true.

Step 3: Evaluate the relationship between the statements.

The relative velocity is the same for both near and distant objects and does not depend on distance.

v⃗rel=−v⃗1\vec{v}_{\text{rel}} = -\vec{v}_1

Since the explanation for Statement-1 relies on the distance-dependent angular velocity ω≈v1D\omega \approx \frac{v_1}{D}, Statement-2 is not the correct explanation for Statement-1.

Answer: Statement-1 is true, Statement-2 is true; Statement-2 is not a correct explanation for Statement-1.

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