Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (4th Edition)

The energy stored in the spring when it is compressed is equal to the kinetic energy of the ball when the ball is released. We can find an expression for the speed $v_0$. $KE_0 = U_0$ $\frac{1}{2}mv_0^2 = \frac{1}{2}kx_0^2$ $v_0^2 = \frac{kx_0^2}{m}$ $v_0 = \sqrt{\frac{kx_0^2}{m}}$ We can find an expression for the speed $v_2$ after the spring is compressed; $2x_0$ $KE_2 = U_2$ $\frac{1}{2}mv_2^2 = \frac{1}{2}k(2x_0)^2$ $v_2^2 = 4\times ~\frac{kx_0^2}{m}$ $v_2 = 2\times ~\sqrt{\frac{kx_0^2}{m}}$ $v_2 = 2~v_0$ The speed increases by a factor of 2.