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The velocity of the centre of mass of a system changes only if there is:
AA net external force
BAny internal force
CA change in shape
DA change in colour
Answer & Solution
Correct answer: A. A net external force
1. Internal forces cancel within a system.
2. When more than one object is involved, the system must be defined.
3. The motion of the whole system is analysed from external forces on the system.
4. The velocity of the centre of mass will change only if there is a net external force.
5. Internal forces cannot move the centre of mass.
_Source: OpenStax College Physics for AP Courses 2e (CC BY 4.0), Ch 4 'Dynamics: Force and Newton's Laws of Motion', sections 4.1-4.8_
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