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 Δl, 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 Δl = 7 mg/x.

Fig. 1.41.