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Two bodies of masses 2 kg and 4 kg are moving with velocities 20 m/s and 10 m/s toward each other due to mutual gravitational attraction. What is the velocity of their centre of mass?

1. 5 m/s
2. 6 m/s
3. 8 m/s
4. zero

Correct answer: zero

Solution

The motion is caused by mutual gravitational attraction, which is an internal force. In the absence of external force, the velocity of the centre of mass remains constant; if the system was initially at rest, it stays zero.

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