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| 183_notes:system_choice [2014/10/29 16:58] – caballero | 183_notes:system_choice [2018/05/29 21:46] – hallstein | ||
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| + | Section 7.7, 7.8 and 7.9 in Matter and Interactions (4th edition) | ||
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| ===== Choosing a System Matters ===== | ===== Choosing a System Matters ===== | ||
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| $$\Delta E_{person} = -\left(\dfrac{1}{2}mv^2 + mgh \right)$$ | $$\Delta E_{person} = -\left(\dfrac{1}{2}mv^2 + mgh \right)$$ | ||
| - | Here we find that the person experiences an internal energy change that is negative, which makes sense because they are expending energy to lift the box. This system is the only one in which we can find this energy change because it is the only one that include | + | Here we find that the person experiences an internal energy change that is negative, which makes sense because they are expending energy to lift the box. This system is the only one in which we can find this energy change because it is the only one that includes |
| Notice that in this system, the person does no work on the box because the box and person are in the system. Instead, we are accounting for the change in energy of the person. | Notice that in this system, the person does no work on the box because the box and person are in the system. Instead, we are accounting for the change in energy of the person. | ||
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| === System 2: Box only === | === System 2: Box only === | ||
| - | Because the system is only a single particle, there' | + | Because the system is only a single particle, there can be no potential energy. There is work done by both the person and the Earth, but they have opposite signs because the applied force and the gravitational force are in different directions. |
| * System: Box; Surroundings: | * System: Box; Surroundings: | ||