Relative Motion in One Dimension

Practice Questions:

Question 11: easy

A car is moving with velocity of 20 m/s on a straight road. A scooterist wishes to overtake the car in 60 s. If the car is at a distance of 1.5 km ahead, then the velocity with which the scooterist has to chase the car is

1. 25 m/s

2. 20 m/s

3. 45 m/s

4. 50 m/s

View Answer

Relative velocity required to cover 1500 m in 60 s is \(v_{\text{rel}} = \frac{1500}{60} = 25\text{ m/s}\). Since \(v_{\text{rel}} = v_s - v_c ⇒ v_s = v_c + v_{\text{rel}} = 20 + 25 = 45\text{ m/s}\).

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Question 12: 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 13: 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 \(\vec{V}_{AB} = \vec{V}_A – \vec{V}_B\).

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): The relative velocity is \(\vec{V}_{AB} = \vec{V}_A - \vec{V}_B\). If \(\vec{V}_B)\) is in the opposite direction to \(vec{V}_A\), then \(|vec{V}_{AB}| = |vec{V}_A| + |vec{V}_B|\), which is greater than (\|vec{V}_A|\). Thus, (A) is False.

Reason (R): The definition of relative velocity of A with respect to B is \(\vec{V}_{AB} = \vec{V}_A - \vec{V}_B\). 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 14: easy

A car is moving with velocity of \( 20\text{ m/s} \) on a straight road. A scooterist wishes to overtake the car in \( 60\text{ s} \). If the car is at a distance of \( 1.5\text{ km} \) ahead, then the velocity with which the scooterist has to chase the car is

1. \( 25\text{ m/s} \)

2. \( 20\text{ m/s} \)

3. \( 45\text{ m/s} \)

4. \( 50\text{ m/s} \)

View Answer

Relative velocity required: \( v_{\text{rel}} = \frac{\text{distance}}{\text{time}} = \frac{1500\text{ m}}{60\text{ s}} = 25\text{ m/s} \). Since \( v_{\text{rel}} = v_s - v_c \), we get \( v_s = v_c + v_{\text{rel}} = 20 + 25 = 45\text{ m/s} \).

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Question 15: easy

In the following question, a statement of Assertion (A) is followed by a statement of Reason (R).

Assertion (A): If two particles, moving along straight line with constant velocities have to meet, the relative velocity must be along the line joining the two particles.

Reason (R): Relative motion means motion of one particle as viewed from the other particle.

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

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

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

4. Both (A) & (R) are false statements

View Answer

For two particles to meet, the relative velocity vector must align with the line joining them so that from one's frame, the other moves directly towards it.

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