A block A of mass 3 kg moving with a speed 2 m/s collides elastically with a stationary block B of mass 4kg. The location of block B is at origin at the time of collision. Assume the collision with th

Question

A block A of mass 3 kg moving with a speed 2 m/s collides elastically with a stationary block B of mass 4kg. The location of block B is at origin at the time of collision. Assume the collision with the wall to be elastic in nature and the size of the blocks is negligible, the x-coordinate of block B when it collides with block A for the second time,

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Elastic collision: v A ​ = 7 3 − 4 ​ ( 2 ) = − 7 2 ​ m / s , v B ​ = 7 2 ( 3 ) ​ ( 2 ) = 7 12 ​ m / s . B reaches the wall at t = 12/7 12 ​ = 7 s and rebounds at − 7 12 ​ m / s ; meanwhile A is at x = − 2 m . Closing speed = 7 12 ​ − 7 2 ​ = 7 10 ​ m / s over 14 m : t ′ = 10/7 14 ​ = 9.8 s . Second collision at x = 12 − 7 12 ​ ( 9.8 ) = 12 − 16.8 = − 4.8 m . Answer: (B)

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