1. The problem statement, all variables and given/known data The collar has a mass of 30 kg and is supported on the rod having a coefficient of kinetic friction of 0.4. The attached spring has an unstretched length of 0.2 m and a stiffness of k =50 N/m. Determine the speed of the collar after the applied force F= 200N causes it to be displaced x=1.5 m from point A. When x=0 the collar is held at rest. Neglect the size of the collar in the calculation. 2. Relevant equations 3. The attempt at a solution I already did the summation of forces in the y direction and found the normal force and used that to find the frictional force. What I'm really confused about is how to know what length to use for the spring. If someone could explain to me why they use certain lengths for certain potential energies it would help me a lot! I also don't understand why they subtracted 0.5 from 0.2 to get the movable end?