What Is the Law of Conservation of Energy?

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For example, consider a $2.0\,\text{kg}$ object falling $5.0\,\text{m}$ without air resistance. Its decrease in gravitational potential energy is

This $98\,\text{J}$ becomes kinetic energy:

so

The energy has changed form, but the total remains $98\,\text{J}$.

A common misconception is that energy is “lost” when friction acts. Energy is not destroyed; it is dissipated into less useful forms, mainly the internal energy of the object and surroundings.

Exam Technique

In Topic A.3, a Calculate question requires the equation, substitution, and numerical answer. For an Explain question, identify the initial and final energy stores, describe the transfer, and state that total energy is conserved. Do not claim that mechanical energy is always conserved; that is true only when resistive forces do not transfer energy out of the mechanical stores.