A ball X moves along a horizontal frictionless surface and collides with another ball Y, as illustrated in Fig. 4.1. [Figure 4.1] Ball X has mass 0.300kg and initial velocity vx at an angle of 60.0° to line AB. Ball Y has mass 0.200kg and initial velocity 6.00 ms⁻¹ at an angle of 60.0° to line AB. The balls stick together during the collision and then travel along line AB, as illustrated in Fig. 4.2. [Figure 4.2] The two balls continue moving together along the horizontal frictionless surface towards a spring, as illustrated in Fig. 4.3. [Figure 4.3] The balls hit the spring and remain stuck together as they decelerate to rest. All the kinetic energy of the balls is converted into elastic potential energy of the spring. The energy E stored in the spring is given by E = ½kx² where k is the spring constant of the spring and x is its compression. The spring obeys Hooke's law and has a spring constant of 72Nm⁻¹.