In a closed system, the following two assumptions are true:
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ME = ME': This means that mechanical energy is conserved in a closed system—however, it's important to clarify that this statement only holds true in the absence of non-conservative forces like friction. In a closed and isolated system with no external forces acting on it, mechanical energy (potential + kinetic energy) remains constant.
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No outside forces act on the system: A closed system is distinguished from an open system by the absence of external forces (or energy) acting on it. In a closed system, the interactions are only among the components within the system itself.
The other options, "Friction is present in the system" and "At least one outside force acts on the system," would not apply to a closed system as defined in classical physics.
So, the two true assumptions are:
- ME = ME' (with the context of no non-conservative forces).
- No outside forces act on the system.
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