Question

Two blocks of masses M and 3M are placed on a horizontal, frictionless surface. A light spring is attached to one of them, and the blocks are pushed together with the spring between them. A cord initially holding the blocks together is burned; after this, the block of mass 3M moves to the right with a speed of 2.00 m/s.

a). What is the speed of the block of mass M?

b). Find the original elastic potential energy in the spring if M = 0.350kg.

Answer 1

a)

$$3M(2)+Mv_B=0\\v_B=-6\frac{m}{s}$$

b)

$$K=0.5(3M)v_A^2+0.5(M)v_B^2\\ K=0.5(3(0.35))(2)^2+0.5((0.35))6^2=8.4\ J$$