(a) Draw an energy diagram for the situation shown in Figure 9.11 for the system that comprises Earth and the box. Assume the box keeps moving at constant velocity, and consider the rope to be part of the person.

(b) Explain each bar in the energy diagram of Figure 9.11b.

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(a) Figure 9.11 If you need to do energy accounting, don't choose a system for which friction occurs at the system boundary. System=person + box System = person + box + Earth (b) Friction generates thermal energy in Earth and box... ... but we don't know how much ends up in Earth and how much in box. If system also includes Earth, all thermal energy remains in system. Earth Earth So, if system excludes Earth, we can't do energy accounting. v Earth Earth so energy accounting is easy. v AK AU AEth AK AU AE, AEth W W