A block is fixed on the extremity of a mobile ideal spring that is horizontal and which has the other side of it fixed (Non mobile). When the lenght of the spring is 50 cm, the block has a force of 5 N on the right; When the lenght of the spring is 80 cm, the block has a force of 10 N vers the left. What is the natural lenght of the spring and what is the spring constant?
Image of the situation : http://imgur.com/RN7Hv3T
The Attempt at a Solution
I know that the natural lenght is going to be between 50 and 80 cm and that the natural lenght is going to be closer to 50 cm because the force is 5 N which indicates that we don't put a lot of pressure on it. Other than that, I have no idea where to go next.