Newton showed us that the gravitational attractiveness of a body depends on its mass. We’ve also learned that a heavy object produces a stronger gravitational field than a lighter one. Our joke about midnight snacks notwithstanding, we’ve been treating mass as something that is constant and does not change. But surely, you must be saying, you can take your blueberry scone and split it in two, thus producing two lighter blueberry scones.
True enough, but if you were to add the mass of the two smaller scones (along with all the crumbs that fell on the floor in the process), you would arrive at exactly the same total mass you started with. To describe this effect, physicists have coined a fancy term: the conservation of mass. It simply states that in any closed system, mass can be neither created nor destroyed. By “closed” we simply mean that all the bits have to be included (e.g., all those crumbs on the floor beneath you).
The physicists love conservation laws so much that they’ve developed lots of them. Another important variant is the conservation of energy. Imagine for a second that, contrary to what we learned above, the moon suddenly asserts its free will and slams on the brakes—halting its orbital motion. For an instant it will enjoy floating motionlessly, high above Earth. Eventually, though, it will begin slowly accelerating toward Earth. It will gradually pick up speed, faster and faster, as it plummets. Then, sadly, it would crash into Earth at great speed.
DEFINITION
In general, a conservation law states that a certain physical property (e.g., mass or energy) for a closed physical system remains unchanged no matter what chemical or physical processes may act on the system. The property may take various subforms (e.g., kinetic energy and potential energy), but their sum total must remain constant.
Throughout the moon’s descent, its total energy was shared between two different forms. When it slammed into Earth at high speed, it had a lot of kinetic energy—a measure of its energy of motion. The moment it halted its orbit, all of its energy was in the form of gravitational potential energy. Most important, at each instant during its fall, the sum of its kinetic and potential energies was fixed and its total energy remained constant. Conservation of energy tells us that energy is neither created nor destroyed, even though it may change from one form to another.
A third conservation law that will be important throughout is the conservation of electric charge. The gravitation relies on mass just as electricity and magnetism rely on something known as electric charge. For now, suffice it to say that, like energy and mass, electric charge can neither be created nor destroyed.
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