# NCERT Class 11 Physics Solutions: Chapter 3 – Motion in a Straight Line Part 4

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**Question: 3.7**

Two trains A and B of length each are moving on two parallel tracks with a uniform speed of in the same direction, with A ahead of B. The driver of B decides to overtake A and accelerates by . If after the guard of B just brushes past the driver of A, what was the original distance between them?

**Answer:**

For train A:

Initial velocity

Time,

Acceleration, Since it is moving with a uniform velocity)

From second equation of motion, distance covered by train A can be obtained as

For train B:

Initial velocity,

Acceleration,

Time,

From second equation of motion, distance covered by train A can be obtained as:

Hence, the original distance between the driver of train A and the guard of train B

**Question 3.8:**

On a two-lane road, car A is travelling with a speed of . Two cars B and C approach car A in opposite directions with a speed of each. At a certain instant, when the distance AB is equal to AC, both being, B decides to overtake A before C does. What minimum acceleration of car B is required to avoid an accident?

**Answer:**

Velocity of car

Velocity of car

Velocity of car

Relative velocity of car B with respect to car A,

Relative velocity of car C with respect to car A,

At a certain instance, both cars B and C are at the same distance from car A i.e.,

Time taken (t) by car C to cover

Hence, to avoid an accident, car B must cover the same distance in a maximum of.

From second equation of motion, minimum acceleration (a) produced by car B can be obtained as: