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Understanding the Law of Conservation of Energy

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  1. The definition of the law of conservation of energy

    The law of conservation of energy is a fundamental principle of physics stating that energy is never newly created or completely destroyed β€” it merely changes form, among kinetic energy, potential energy, heat energy, electrical energy, and others β€” while its total amount always remains constant.

  2. Energy conversion in a pendulum

    At a pendulum's highest point, its speed is zero and its potential energy is at its maximum; as it passes through its lowest point, its speed is at its maximum and its kinetic energy is at its maximum. Assuming no air resistance or friction, as it swings back and forth between these two points, potential energy and kinetic energy simply change form into one another, while the total mechanical energy β€” the sum of the two β€” always remains the same.

  3. Energy conversion on a roller coaster

    A roller coaster stores the maximum amount of potential energy at the top of its first, highest hill, and as it descends, that potential energy converts into kinetic energy, increasing its speed. For this reason, ride designers make each subsequent hill lower than the first, so the car can clear each following section using only its remaining energy.

  4. The relationship to the first law of thermodynamics

    The first law of thermodynamics expresses the law of conservation of energy in terms of heat and work: heat energy added to a system is used either to raise the system's internal energy or to do work on its surroundings, and the total amount is conserved. In other words, the law of conservation of energy is a broader principle that applies not just to mechanics but to all forms of energy in common β€” heat, chemistry, electricity, and more.

  5. Why perpetual motion machines are impossible

    A perpetual motion machine β€” one that moves on its own forever and continues doing work without any external energy supply β€” cannot be built, because it would violate the law of conservation of energy (and, due to friction, the second law of thermodynamics as well). Every real machine loses some energy, which eventually dissipates as heat through friction or air resistance, so motion will gradually stop unless energy continues to be supplied from outside.

  6. The various forms of energy

    Beyond kinetic and potential energy, energy exists in many other forms β€” heat energy, chemical energy (stored in food or fuel), electrical energy, light energy, nuclear energy, and more. A car, for example, sequentially converts the chemical energy of its fuel into heat energy through combustion, and then into kinetic energy through the engine.

Why is the law of conservation of energy a core principle of physics?

The law of conservation of energy is one of the most fundamental principles running through nearly every field of physics and chemistry β€” mechanics, thermodynamics, electromagnetism, chemistry, and more. Simply examining "where energy moved from and to" when analyzing a given phenomenon can explain a great many things.

Also explore the principles behind the motion that energy creates

The actual motion produced as energy converts from one form to another can also be explained through the relationship between force and acceleration. See our guide to understanding Newton's 3 laws of motion to explore the relationship between force and motion.

Frequently Asked Questions

Why does it sometimes seem like energy disappears?

Even when kinetic energy appears to decrease due to friction or air resistance, that energy hasn't actually disappeared β€” it has simply changed into a less noticeable form, such as heat energy or sound energy. The total amount of energy overall is always conserved.

Do nuclear fission and nuclear fusion also follow the law of conservation of energy?

Yes, though in this case, it's more accurate to say that the combined total of mass and energy is conserved, since some of the mass converts directly into energy according to Einstein's mass-energy equivalence principle (E=mcΒ²).