3.4.A.1–2Concept
A system made of only a single object can have only kinetic energy (3.4.A.1). A system that contains objects interacting through conservative forces, or that can change its shape reversibly, may have both kinetic and potential energy (3.4.A.2).
ExampleOne object versus two1 / 3
A 2 kg ball is 3.0 m above the ground and moving at 5 m/s. Take U = 0 at the ground. (a) What energies are present if the ball alone is the system, and how large are they? (b) What energies are present if the system is the ball and Earth?
Try it on paper first, then reveal each step to check your work.
Step 1 — (a) The ball alone
A single-object system can only have kinetic energy. K = ½mv² = ½(2)(5)² = 25 J. There is no potential energy, because potential energy needs two or more interacting objects.