Motion of Centre of Mass — Prelims Strategy

Prelims Strategy

To excel in NEET questions on the motion of the center of mass, adopt a systematic approach. First, clearly define the 'system' under consideration. This is crucial for correctly identifying internal and external forces.

Remember that internal forces (like explosion forces, tension between parts of a system, or forces between colliding bodies) do not affect the motion of the center of mass. Only external forces (like gravity, friction, or applied pushes/pulls from outside the system) can change the CM's velocity or acceleration.

For numerical problems, always use vector notation for position, velocity, and acceleration. Pay close attention to the signs and components. When dealing with conservation of momentum problems (where net external force is zero), remember that the velocity of the center of mass remains constant.

This is particularly useful for problems involving explosions or a person moving on a boat. For conceptual questions, focus on the core principle: $\vec{F}_{ext} = M \vec{A}_{CM}$. Trap options often involve confusing internal and external forces or assuming the CM follows the path of an individual particle.

Practice a variety of problems, especially those involving relative motion and multi-particle systems, to build intuition.