Problem

Two blocks are connected to an ideal spring of stiffness 200 N/m. At a certain moment, the two blocks are moving in opposite directions with speeds

4 \text{ ms}^{-1}

and

6 \text{ ms}^{-1}

, and the instantaneous elongation of the spring is 10 cm. The rate at which the spring energy

\left(\frac{kx^2}{2}\right)

is increasing is a. 500 J/s b. 400 J/s c. 200 J/s d. 100 J/s