Question

A proton and an electron, initially at rest, are allowed to move under their mutual attractive force. Their centre of mass will:

Answer 1

Center of mass of a proton and an electron

1. A proton and an electron exert the same magnitude of electric force on each other owing to their equal but opposite nature of charge.
2. The ratio of their distances to the center of mass will remain the same because the force acting on them is equal. Their acceleration will be different but the unequal mass compensates for that.
3. Thus, the center of mass will not change,
4. Generally, an external force is required to move the center of mass of a system.
5. The mutual force of attraction between a proton and an electron is an internal force, not an external force.

Therefore, the center of mass will not change.