Inertia is the tendency of objects to resist changes in their motion. An object at rest stays at rest, and an object moving in a straight line at constant speed keeps doing so unless a net external force acts on it. This is Newton's First Law, also known as the law of inertia. It helps explain why a puck on a smooth surface keeps gliding in a straight line until friction or another force slows it down or changes its direction. The other ideas describe different concepts: a force causing acceleration is Newton's Second Law; gravity is the attraction between masses; and conservation of energy is about energy staying the same in a system, not about motion persisting without a net force.

Mass is conserved in chemical reactions: the total amount of matter before a reaction is the same as after. Atoms are simply rearranged to form new substances, but they aren’t created or destroyed, so the total mass doesn’t change. This idea is called the conservation of mass. It’s why chemical equations are balanced: the same number and kind of atoms appear on both sides, ensuring the mass is the same before and after. The other terms describe parts of matter—the nucleus, the electron, and the atom—but they don’t express the rule that mass stays constant during a reaction.

Energy is the ability to cause change, or to do work. When you apply a force to move something, you’re using energy to produce that movement or other change. The amount of energy something has tells you how much work it can do or how much it can change things, now or later. The rate at which energy is used or transferred is called power, so saying “rate of performing work” describes power, not energy itself. Energy isn’t just the force that moves objects; it’s the capacity behind that force to cause motion or other changes. And energy isn’t only motion energy; it includes many forms like potential energy, chemical energy, electrical energy, and more, all of which can be transformed from one form to another.

Energy in a simple machine is conserved and can move between stored energy and motion without creating or destroying it. As something moves through a simple machine, its energy can switch between potential energy (stored energy due to position) and kinetic energy (energy of motion). For example, an object at the top of a ramp has more potential energy, and as it slides down, that potential energy becomes kinetic energy while the total amount stays the same (ignoring friction). If friction or other losses are present, some energy still ends up as heat or sound, but the overall energy balance remains constant when all forms are counted.