JEE Challenger
More from Motion in a Straight Line

Apparent Motion of Nearby and Distant Objects from Moving Train

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.
and
STATEMENT-2
If the observer and the object are moving at velocities V⃗1\vec{V}_1 and V⃗2\vec{V}_2 respectively with reference to a laboratory frame, the velocity of the object with respect to the observer is V⃗2−V⃗1\vec{V}_2 - \vec{V}_1.

Options

A

STATEMENT-1 is True, STATEMENT-2 is True; STATEMENT-2 is a correct explanation for STATEMENT-1

B

STATEMENT-1 is True, STATEMENT-2 is True; STATEMENT-2 is NOT a correct explanation for STATEMENT-1

Correct
C

STATEMENT-1 is True, STATEMENT-2 is False

D

STATEMENT-1 is False, STATEMENT-2 is True

Step-by-Step Solution

To determine the correct choice, let us analyze both statements individually:

1. Analysis of Statement-1:
Consider a train moving with velocity V⃗train=Vi^\vec{V}_{\text{train}} = V\hat{i} relative to the ground. For any stationary object on the ground (V⃗object=0⃗\vec{V}_{\text{object}} = \vec{0}), its relative velocity with respect to the observer inside the train is:
V⃗rel=V⃗object−V⃗train=−Vi^\vec{V}_{\text{rel}} = \vec{V}_{\text{object}} - \vec{V}_{\text{train}} = -V\hat{i}

Thus, both nearby and distant objects have the same relative velocity vector relative to the observer. However, the visual perception of motion by the human eye depends on the rate of change of the angle of sight, which is quantified by the angular velocity ω\omega:
ω=V⊥r\omega = \frac{V_{\perp}}{r}
where rr is the distance from the observer to the object and V⊥V_{\perp} is the component of relative velocity perpendicular to the line of sight.

  • For nearby objects, the distance rr is small, resulting in a large angular velocity ω\omega. Hence, nearby objects appear to move rapidly past the observer in the opposite direction.
  • For distant objects, the distance rr is extremely large, making the angular velocity ω≈0\omega \approx 0. Consequently, distant objects appear to be practically stationary.

Therefore, Statement-1 is True.


2. Analysis of Statement-2:
By the fundamental definition of relative velocity in classical kinematics, if an observer moves with velocity V⃗1\vec{V}_1 and an object moves with velocity V⃗2\vec{V}_2 relative to a reference (laboratory) frame, the velocity of the object relative to the observer is given by:
V⃗relative=V⃗2−V⃗1\vec{V}_{\text{relative}} = \vec{V}_2 - \vec{V}_1

Therefore, Statement-2 is True.


3. Relationship between Statement-1 and Statement-2:
Statement-2 states that the relative linear velocity of all stationary objects (V⃗2=0⃗\vec{V}_2 = \vec{0}) with respect to the moving observer (V⃗1=V⃗train\vec{V}_1 = \vec{V}_{\text{train}}) is identical, namely −V⃗1-\vec{V}_1, irrespective of their distance from the observer.

Because Statement-2 gives the same linear relative velocity for both nearby and distant objects, it fails to explain why distant objects appear stationary while nearby objects appear to move fast. The actual explanation lies in the difference in angular velocity (ω=V⊥/r\omega = V_{\perp}/r) subtended at the observer's eye due to differing distances rr.

Hence, Statement-2 is NOT the correct explanation for Statement-1.

Correct Answer: B (STATEMENT-1 is True, STATEMENT-2 is True; STATEMENT-2 is NOT a correct explanation for STATEMENT-1)