Work, Energy & Power
Class 11 · Work, Energy & Power
Positive vs Negative Work
Switch force angle: 0° → +W, 180° → −W (friction), 90° → 0. See the sign change.
Key Notes
01
W = F·d cosθ. Sign depends entirely on the angle between force and displacement.
02
θ = 0° → +W (force aids motion). θ = 180° → −W (force opposes motion, e.g. friction).
03
θ = 90° → W = 0 (centripetal force, normal force on a horizontal floor — perpendicular).
Formulas
Work
Sign of cosθ determines sign of W.
Important Points
Friction always does negative work on a sliding object (relative to ground).
Tension in a string passing over a smooth pulley does zero net work on the system.
Positive vs Negative Work notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 11 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.