Lesson 5 of 10 · 7 min
Potential energy and conservative forces
NCERT § "The Concept of Potential Energy"
A 1.5 kg coconut hangs 20 m up the palm beside the stall. It isn't moving, yet everyone stands clear. What is stored in it, and why is it measured from the ground and not from the stall's 5 m roof?
The lesson in notes
In short
Potential energy is energy stored by virtue of a body's position or configuration. It is defined only for conservative forces.
A force is conservative if the work it does depends only on the start and end points, not on the path; equivalently, the work it does over any closed path is zero.
Gravity near the earth's surface and the spring force are conservative. Friction and air drag are not, since the work they do depends on the path.
For a conservative force in one dimension, F(x) = −dV/dx: the force points towards decreasing potential energy.
Near the earth's surface the gravitational potential energy of a mass m at height h is V(h) = mgh, measured from a chosen zero level.
Only changes in potential energy have physical meaning; the zero level can be chosen wherever convenient.
Potential energy has the same unit as work and kinetic energy, the joule.