3.4.A.1–2Concept
A system composed of only a single object can only have kinetic energy. A system that contains objects interacting through conservative forces, or that can change shape reversibly, may have both kinetic and potential energy.
ExampleA single object vs. a system of two1 / 3
A 2 kg ball flies through the air at 8 m/s, 5 m above the ground. (a) If the system is defined as just 'the ball,' what type(s) of energy can it have? (b) If the system is instead defined as 'the ball + Earth,' what type(s) of energy can it have? (c) Find the numerical value of each applicable energy for the larger system.
Try it on paper first, then reveal each step to check your work.
Step 1 — System = the ball alone
A single-object system can only have kinetic energy (3.4.A.1) — there's no second object to interact with, so no potential energy is possible.