Britannica Sources used in this answer

Why is total energy conserved even when an object slows to a stop?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

A diagram with labeled vector illustrations. Panel A shows vector addition with two green arrows A and B forming a parallelogram, and their sum C as a red diagonal. Panel B shows vector subtraction, with A and negative B forming resultant D. Panel C demonstrates vector components using angles and projections. Panel D uses the right-hand rule with thumbs to show the direction of the cross product E = A x B and its opposite, -E = B x A. — mechanics Figure 1: (A) The vector sum C = A + B = B + A. (B) The vector difference A + (−B) = A − B = D. (C, left) A cos θ is the component of A along B and (right) B cos θ is the component of B along A. (D, left) The right-hand rule used to find the direction of E = A × B and (right) the right-hand rule used to find the direction of −E = B × A.

Total energy is conserved when an object slows to a stop because energy is never created or destroyed; it merely changes form. While it may appear that energy is lost when an object stops, it has actually been converted into other types of energy.12

When an object slows down, its kinetic energy—the energy of motion—is transformed into other forms depending on the physical context:12

Potential Energy: If an object slows while moving against a force like gravity (such as a pendulum swinging upward), its kinetic energy is converted into potential energy.12
Thermal Energy: If an object is brought to a stop by friction, its kinetic energy is dissipated as thermal energy (heat).12

In any closed system, the sum of all energy forms—including kinetic, potential, and thermal—remains constant. This principle, known as the conservation of energy, is a fundamental law of thermodynamics.12