What's the Difference Between Kinetic Energy, Gravitational Potential Energy, and Elastic Potential Energy?

For SL and HL: Kinetic energy is associated with motion, gravitational potential energy with position in a gravitational field, and elastic potential energy with the deformation of an elastic object. All three are measured in joules and can be transformed into one another.

The Differences

Here, $m$ is mass, $v$ is speed, $g=9.8\,\text{m s}^{-2}$, $\Delta h$ is the vertical change in height, is the spring constant, and is the extension or compression from equilibrium.

These categories depend on the chosen system and reference level. Kinetic energy depends on the observer's reference frame because measured speed can differ. Gravitational potential energy is a change relative to a selected zero height, so only $\Delta E_p$ is physically important in near-Earth calculations. Elastic potential energy assumes Hooke's law applies, meaning extension is proportional to restoring force. In each case, work done transfers energy into or out of the relevant store.

For example, consider a $2.0\,\text{kg}$ object moving at $3.0\,\text{m s}^{-1}$, raised by $5.0\,\text{m}$, or attached to a $200\,\text{N m}^{-1}$ spring extended by :

• $E_k=\frac{1}{2}(2.0)(3.0)^2=9.0\,\text{J}$

Their sum is the system's mechanical energy. Without friction or other resistive forces, total mechanical energy is conserved. With resistance, mechanical energy decreases because energy is transferred to thermal energy, while total energy remains conserved.

A common misconception is that energy is “lost.” Energy is not destroyed; it is transferred or dissipated into less useful forms.

Exam Technique

In Topic A.3, identify the initial and final energy forms before selecting equations. Measure $\Delta h$ vertically, square both $v$ and $\Delta x$, and state clearly whether mechanical energy is conserved.