UPSC NDA 2 2019 General Ability Solved Paper

Concept:The centre of mass of a system moves only under the influence of external forces.Internal forces, like those from a chemical reaction, cannot alter the velocity of the centre of mass.However, internal forces can change the relative motion of particles inside the system.Explanation:Let the object have mass $M$ and move with uniform velocity $v$.Since no net external force acts on the system, the acceleration of the centre of mass is zero.$F_{\text{ext}} = M a_{\text{cm}} = 0$ implies $a_{\text{cm}} = 0$.Hence the velocity $v$ of the centre of mass remains constant.Thus statement 1 is correct.Chemical reactions generate internal forces.These internal forces can change the velocities of individual particles relative to the centre of mass.For example, an explosion increases the relative kinetic energy of the fragments.Therefore, the kinetic energy of particles with respect to the centre of mass can change.So statement 2 is false.Answer:Option A (1 only)