Two identical balls are suspended: the first on a thread l1 = 0.5 m long, the second - on a thread l2 = 1 m long. When the threads are vertical, the balls touch each other, and the line connecting their centers is horizontal. If the first ball is deflected by an angle of 15 degrees; and then let go, then the second after the collision deviates by an angle of 8 degrees, and at the moment of the collision the first thread breaks. What is the nature of the collision (absolutely elastic, absolutely inelastic, partially elastic)?

A detailed solution with a brief note of the conditions, formulas and laws used in the solution, the derivation of the calculation formula and the answer.