一、The figure below shows a block with mass m = 0.400 kg sits on a frictionless air track, connected to a spring with force constant k = 10.00 N/m (equilibrium position, x = 0). You pull on the block, stretching the spring 0.100 m} (x = 0.100), and release it from rest. The block moves back toward its equilibrium position. What is its x-velocity when x = 0.080 m? 【Hint】: 1. Hooke's law: $F_{x} = -kx$ . 2. Work by a spring force: $W_{s} = \frac{1}{2} kx_{i}^{2} - \frac{1}{2} kx_{f}^{2}$ .