posted by jeff .
A 5 kg box is pushed against a K= 40 n/m spring on a horizontal surface and the spring is compressed 0.12m. The box is released and travels for 1.4 meters before comin to a stop. What is the coefficent of friction between the floor and the box. How fast was the box moving when it lost contact with the spring
I suggest you try an energy conservation approach. The work done against friction will equal the initial system total energy. From that you can deduce the friction force and coefficient.
The kinetic energy when contact with the spring is lost equals the initial stored spring potential energy, less a small amount for friction work while moving 0.12 m