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# In the absence of external force acting n a rigid system, which of the following must remain constant 1) angular momentum 2) linear momentum 3) moment of inertia 4) kinetic energy Which one is the correct answer ?

$a)\; 1\; and\; 2\\b)\; 2\; and\; 4 \\c)\;3 \;and\; 4 \\d)\; 1\; and\; 4$

In the absence of external forces , angular momentum and linear momentum of system is conserved .
Moment of inertia may change.
kinetic energy may also change say like in an inelastic collision.