Two blocks, of masses $2.0\ \text{kg}$ and $3.0\ \text{kg}$, rest on a smooth floor, joined by a light spring of force constant $100\ \text{N/m}$ at its natural length. A constant horizontal force of $15\ \text{N}$ starts to pull the $3.0\ \text{kg}$ block away from the other one, as shown in the figure. What is the largest stretch of the spring?

Fig. 1: On a smooth floor, a block of mass 2.0 kg on the left is joined by a spring of spring constant k = 100 N/m, at its natural length, to a block of mass 3.0 kg on the right. A force F = 15 N pulls the 3.0 kg block to the right.