A 1kg block is at rest on a horizontal frictionless surface touching the end of an unstretched spring of spring constant 200N/m. Tjhe other end of the spring is fixed to a wall. A 2kg block traveling at a speed v(initial)=4m/s collides with the 1kg block, and the two blocks stick together. I am not sure if momentum or kinetic energy is conserved in this collision, if it is please explain. When the blocks momentarily stop, by what amount is the spring compressed? I understand that when the spring expands it will accelerate the coupled blocks from rest so what is the speed of the blocks when they are 0.20m to the left of the spring's end?