how much energy??

Question

Two blocks of respective masses m1=0.35 kg and m2=7.5 kg stand without motion on a frictionless horizontal table. A spring is compressed between the two blocks, but the blocks remain motionless because they are tied to each other by a cord. The masses of the spring and the cord are not negligible compared to the masses of the blocks. Once the system is set up, the cord is burned by a match and the blocks are pushed apart by the spring. Once the spring is completely relaxed, it detaches from the m1 block, and the blocks continue to move away from each other at respective speeds v1 and v2. Given m1=0.35 kg, m2= 7.5 kg, v2= 5.9 m/s but v1 is not given. How much energy was stored in the compressed spring before the cord was burned?

Answer 1

You explicitly say that the mass of the spring and the mass of the cord are NOT negligible, but you don't give them. That makes solving this problem impossible.

If those masses were negligible, then you could use conservation of linear momentum which says:
m1 v1 = m2 v2

With three of the numbers, you could compute the fourth.

Then energy conservation gives you:

initial spring energy = E1 + E2

E1 = (1/2) m1 v1^2
E2 = (1/2) m2 v2^2

but with mass missing, none of this works.