Positive, Negative, and Zero Work
The sign of work depends on the angle between force and displacement: work is positive when both are in the same direction, negative when opposite, and zero when perpendicular or when displacement is absent.
Practice This ConceptMain explanation
Teacher explanation
Positive work increases or supports motion, like gravity doing work on a falling stone. Negative work opposes motion, like friction on a sliding box. Zero work occurs when force does not produce displacement in its direction, such as centripetal force in uniform circular motion at an instant.
Example
When a ball moves upward, gravity acts downward, so work done by gravity is negative.
Simple analogy
Same helps, opposite resists, perpendicular does no work.
Common confusion
Students often say work is negative only when the object moves backward, but it actually depends on force direction relative to displacement.
Exam tip
Draw force and displacement arrows. Same direction gives positive work, opposite direction gives negative work, and perpendicular gives zero work.
Answer writing and exam use
1-mark use
Write the exact meaning of positive, negative, and zero work in one clean line.
2-mark use
Define positive, negative, and zero work and add one example or condition.
3-mark use
Explain positive, negative, and zero work, show the method or example, and mention the common mistake.
Practice this concept with focused MCQs
Open the concept quiz intro first, review the test details, and then start a focused MCQ set from this concept only. Instant score and answer review are live now.
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