4. A block of mass m is attached to a wall by a massless spring, as shown. The spring is initially neither stretched nor compressed, and there is no friction between the block and the surface. At time t, a string begins pulling the block to the right with a constant force. The block moves a distance d before turning around and returning to the left; the block reaches the turnaround point at time 12. In this problem, you will analyze this situation by considering two different systems: system SB (the spring and the block) and system B (the block alone). t=t a. x=0 t=1 w For both systems, consider the interval from ti to tz. Consider the system SB: the spring and the block. i. For each horizontal force that is acting on system SB, specify whether the work done by the force is positive, negative, or zero. Explain your reasoning. (Hint: There are two horizontal forces.) x=0 ii. Complete the energy bar diagram for system SB at right. If any of the quantities is zero, state so explicitly. Explain your reasoning. x=d ii. Complete the energy bar diagram for system B at right. If any of the quantities is zero, state so explicitly. Explain your reasoning. x=d System SB AK AU W net ext b. Consider the system B: the block alone. i. For each horizontal force that is acting on system B, specify whether the work done by the force is positive, negative, or zero. Explain your reasoning. System B AK AU Wnet ext