Two blocks of masses M and 2M are connected to a light spring of constant K as shown. The surface and pulley are frictionless. They are released from rest when the spring is undeformed. Consider: (I) maximum extension=4Mg/K; (II) maximum kinetic energy=2M²g²/K; (III) maximum extension=2Mg/K; (IV) maximum kinetic energy=4M²g²/K. Which option is correct?

Question

G1 Physics · Accepted Answer

At maximum extension x, the system is momentarily at rest: 2Mgx=½Kx², giving x=4Mg/K, so I is true. Maximum kinetic energy occurs where Kx=2Mg, i.e. x=2Mg/K; then K(max)=2Mgx-½Kx²=2M²g²/K, so II is true. Hence B.