Problem 1.153

A system consists of two identical cubes, each of mass $m$ linked together by the compressed weightless spring of stiffness $x$ (Fig. 1.41). The cubes are also connected by a thread which is burned through at a certain moment. Find: (a) at what values of $\Delta l$, the initial compression of the spring, the lower cube will bounce up after the thread has been burned through: (b) to what height $h$ the centre of gravity of this system will rise if the initial compression of the spring $\Delta l=7 \mathrm{mg} / \mathrm{x}$