Two blocks of masses M and 3 M are placed on ahorizontal, frictionless surface.

Question

Two blocks of masses M and 3M are placed on ahorizontal, frictionless surface. A light spring is attached to oneof them, and the blocks are pushed together with the spring betweenthem, as seen in the figure below. A cord initially holding theblocks together is burned; after this the block of mass 3Mmoves to the right with a speed ofv1=2.06 m/s. Calculate the originalelastic energy in the spring if M = 0.300 kg.

Explanation / Answer

let us consider the two masses and the spring in to onesystem then there are no other forces other than the the internal forceson each block so we can conserve the momentum the momentum of larger block = momentum of the smaller block =>3M*2.06=M*v v=6.18m/sec the energy is conserved in this collision as no non conservativeforces are involved hence initial spring energy= finalK.E=0.5[0.9*2.06*2.06+0.3*6.18*6.18]J

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