The variation with extension x of the force Facting on a spring is shown in Fig. 3.1. [Figure 3.1] The spring of unstretched length 0.40 m has one end attached to a fixed point, as shown in Fig. 3.2. [Figure 3.2] A block of weight 2.5 N is then attached to the spring. The block is then released and begins to move downwards. At one instant, as the block is continuing to move downwards, the spring has a length of 0.72 m, as shown in Fig. 3.3. [Figure 3.3] Assume that the air resistance and the mass of the spring are both negligible.