1. The problem statement, all variables and given data A mass hangs in balance on a elastic rope (k= 800 N/m). The lenght of the rope in balance is 1m. They pull the mass 25 cm out of balance. The mass colides with the ceiling (restitution coefficient = 0,8). How far extend the rope after the collision. ( Solution: 0,0113m) 2. Relevant equations In balance: m*g=k*u Ek=1/2 m v2 Ep= m*g*h Ev= 1/2 k u2 3. The attempt at a solution I have tried different things with using the law of conversation of energy. But I think a have not enough known date. So, I compared the energy of the moment when the mass is pulled 25cm out of balance with the moment it collides with the ceiling: 1/2 k (u+0,25)2=1/2 m v2+ m g 1,25 I want to determine the speed. Then I know that the speed after the collision is equal to 0,8v. Then I would use the law of conversation of energy again to determine the extending of the rope. TO determine v I think have not enough data but maybe I forgot something. Can anyone help me? Thanks a lot!