Question

A body having velocity of 20 m/s towards +ve x-axis collides with a body initially moving with 10 m/s in -ve x-axis direction. If the collision is elastic and the first body is now moving with a velocity of 15 m/s towards -ve x-axis. The second body must be moving with a velocity of in direction.

• A. negative x-axis
• B. positive x-axis
• C. 5 m/s
• D. 15 m/s
• E. 20 m/s

Answer 1

Answer: (B), (D)

The correct options are
B positive x-axis
D 15 m/s

As the collision is elastic, velocity of separation must be equal to velocity of approach.

As we can see they are approaching with the velocity of 30 m/s and the first body now moves with -15 m/s. Hence after the collision, the second body must move with + 15 m/s to make velocity of separation of 30 m/s which must be the case.