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The work done by nonconservative forces changes the total mechanical energy of a system. When this nonconservative work Wnc is positive, such as a person pushing a crate up a ramp, what happens to the mechanical energy?
AIt stays constant no matter the value of Wnc
BIt always decreases, since Wnc removes energy
CIt increases here, since Wnc adds energy
DIt becomes undefined once Wnc acts on it
Answer & Solution
Correct answer: C. It increases here, since Wnc adds energy
1. The relationship KEi + PEi + Wnc = KEf + PEf shows that nonconservative work adds directly to the system's mechanical energy budget.
2. When Wnc is positive, as when a person pushes a crate up a ramp against friction, the final mechanical energy exceeds the initial mechanical energy, so mechanical energy increases.
3. Mechanical energy is not always constant once a nonconservative force is present, and it remains a perfectly well defined quantity throughout the process.
4. A positive Wnc specifically means energy is being added, the opposite of the claim that nonconservative forces always remove energy.
_Source: OpenStax College Physics (CC BY 4.0), Ch 7 "Work, Energy, and Energy Resources", section 7.5 Nonconservative Forces_
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