What is energy defined as in physics?

• A. The ability to do work, defined as the amount of work a force can perform and constrained by conservation laws.
• B. The rate at which work is done.
• C. The distance an object travels under a force.
• D. The magnetic flux through a surface.

Answer: (A)

Energy is the ability to do work. It is the capacity of a system to perform work, and we quantify it by the amount of work that can be done as energy is transferred or transformed, with the total energy in a closed system remaining constant over time. This is why energy is described as the quantity that can be converted between forms—kinetic, potential, thermal, chemical, etc.—while obeying conservation laws.

The rate at which work is done is not energy but power, so that describes how quickly energy is used. The distance an object travels under a force relates to work (and thus energy) only when the force and displacement align; by itself, distance is not the definition of energy. Magnetic flux is unrelated to the concept of energy in this context.