Two blocks of masses m and 3m are placed on a frictionless, horizontal surface.

Question

Two blocks of masses m and 3m are placed on a frictionless, horizontal surface. A light spring is attached to the more massive block, and the blocks are pushed together with the spring between them. A cord initially holding the blocks together is burned; after that happens, the block of mass 3m moves to the right with a speed of 2.00 m/s. (a) What is the velocity of the block of mass m? (b) Find the system's original elastic potential energy, taking m = 0.350kg. (c) Is the original energy in the spring or in the cord? (d) Explain your answer to part (c). (e) Is the momentum of the system conserved in the bursting-apart process? Explain how that is possible considering (f) there are large forces acting and (g) there is no motion beforehand and plenty of motion afterward?

Explanation / Answer

conservation of energy . 3m*2+m*v=0. so v=-6(m/s) (to the left). ------------ energy is conserved. so K=035*6^2/2+3*0.35*2^2/2=8.4(J) ------------- in the spring, because potential energy in the spring has transfered to kinetic energy of the two masses. The cord has stored no potential energy. ------- Yes. Because it is simple that the two masses are in one isolated system. THe spring is mass less and it has only use is of storing potential energy.

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