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When the total nonconservative work Wnc done on a system is exactly zero, what does this imply about the system's mechanical energy?
AMechanical energy is conserved, forces are balanced
BMechanical energy must be increasing steadily
CMechanical energy must be decreasing steadily
DNo conclusion can be drawn about the energy
Answer & Solution
Correct answer: A. Mechanical energy is conserved, forces are balanced
1. The equation Wnc = delta KE + delta PE shows that nonconservative work directly sets the change in mechanical energy.
2. If Wnc is exactly zero, then delta KE plus delta PE is also zero, so the total mechanical energy stays constant, meaning it is conserved.
3. Zero net nonconservative work does not mean energy is steadily increasing or decreasing, since a zero change is neither of those.
4. This case does allow a clear conclusion, unlike the claim that nothing can be said, since Wnc = 0 pins down that mechanical energy is conserved.
_Source: OpenStax College Physics (CC BY 4.0), Ch 7 "Work, Energy, and Energy Resources", section 7.5 Nonconservative Forces_
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