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The motion of the centre of mass of a system of two particles is unaffected by their internal forces:

1. irrespective of the actual directions of the internal forces
2. only if they are along the line joining the particles
3. only if they are at right angles to the line joining the particles
4. only if they are obliquely inclined to the line joining the particles

Correct answer: irrespective of the actual directions of the internal forces

Solution

Internal forces cancel in pairs according to Newton’s third law, so they do not affect the motion of the centre of mass. Therefore, the centre of mass depends only on external forces.

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