Lesson 2 of 10 · 11 min
Work
NCERT § "Work"
Kiran hauls the trolley 10 m at a steady walk. He is tired, so he has done work. But has the trolley's weight done any? And what about the friction from the dirt road?
The lesson in notes
In short
Work done by a constant force F during a displacement d is W = F·d = Fd cos θ. Only the component of force along the displacement does work.
Work is a scalar; its SI unit is the joule, 1 J = 1 N m. Other units: 1 erg = 10⁻⁷ J, 1 eV = 1.6 × 10⁻¹⁹ J, 1 kWh = 3.6 × 10⁶ J.
No work is done if the displacement is zero (pushing a rigid wall), if the force is zero (a body moving freely at constant velocity), or if force and displacement are perpendicular.
Work can be negative: the force of friction on a sliding body and the gravitational force on a rising body both do negative work.
Always name the force whose work is asked for: the work done by gravity, by friction and by the applied force on the same body are separate quantities.
When several forces act, the net work is the sum of the work done by each, which also equals the work done by the net force.