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Relative Motion in One Dimension

Practice Questions:

Question 1: easy

Assertion (A): If separation between two particles does not change then their relative velocity will be zero.

Reason (R): Relative velocity is the rate of change of position of one particle with respect to another.

1. (1) Both (A) & (R) are true and the (R) is the correct explanation of the (A)

2. (2) Both (A) & (R) are true but the (R) is not the correct explanation of the (A)

3. (3) (A) is true but (R) is false

4. (4) Both (A) and (R) are false

View Answer

Assertion (A): If the separation \(\vec{r}_{rel}\) is constant, it implies \(\vec{r}_{rel} \cdot \vec{v}_{rel} = 0\), meaning \(\vec{v}_{rel}\) is perpendicular to \(\vec{r}_{rel}\), but not necessarily zero (e.g., two particles orbiting each other at constant distance). So (A) is False.

Reason (R): Relative velocity is defined as the time derivative of the relative position vector. So (R) is True.

Since (A) is false and (R) is true, none of the given options are strictly correct. However, if (A) is false, options (1), (2), (3) are ruled out, leaving (4) by elimination, despite (R) being true.

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Question 2: easy

Assertion (A): The magnitude of velocity of A with respect to B will be always less than (V_A).

Reason (R): The velocity of A with respect to B is given by \(\vec{V}_{AB} = \vec{V}_A – \vec{V}_B\).