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Two particles which are initially at rest, move towards each other under the action of their internal attraction. If their speeds are v and 2v at any instant, then the speed of centre of mass of the system will be:

1. 2v
2. zero
3. 1.5
4. v

Correct answer: zero

Solution

The center of mass of a system of particles remains at rest if no external force acts on the system. Since the particles are moving due to internal forces, the speed of the center of mass will remain zero.

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