Forms of mechanical energy
Gravitational and elastic potential energy
Gravitational potential energy
An object at a height possesses gravitational potential energy, denoted Epp, because it could fall and thus release energy. It depends on the mass m, the acceleration due to gravity g (approximately 10 N/kg on Earth) and the height h relative to a chosen reference point:
Epp = m * g * h
Choice of reference level
The value of h (and therefore of Epp) depends on the level chosen as the reference (often the ground, or the lowest point of the trajectory). This choice is arbitrary, but it must remain consistent throughout an exercise.
Example
A book with a mass of 0.8 kg placed on a shelf 1.5 m above the ground has gravitational potential energy: Epp = 0.8 * 10 * 1.5 = 12 J
Elastic potential energy
A spring compressed or stretched by x (in m) relative to its rest position, with stiffness k (in N/m), stores elastic potential energy:
Epe = 1/2 * k * x^2
Pitfalls to avoid
- Forgetting to clearly define the origin of the heights (where is h = 0?).
- Confusing h with the total distance travelled if the trajectory is not vertical.
- Omitting the factor 1/2 in Epe, as with kinetic energy.