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# A set of identical cubical blocks lies at rest parallel to each other along a line on a smooth horizontal surface. The separation between the near surfaces of any two adjacent blocks is . The block at one end is given a speed towards the next one at time . All collision are completely elastic. Then

- The last block starts moving at time
- The last block starts moving at time
- The centre of mass of the system will have a final speed
- The centre of mass of the system will have a final speed

## The correct answer is: The centre of mass of the system will have a final speed

### Time taken by first block to reach second block

Since collision is 100% elastic, now first block comes to rest and 2nd block starts moving towards the 3rd block with a velocity and takes time to reach 3rd block and so on

Total time time

Finally only the last th block is in motion velocity , hence final velocity of centre of mass

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