# Why is Weight Important?

it’s remarkable how much you can understand about the world without worrying about any properties of an object other than its weight

In the scientist’s menagerie of important properties, weight is right up there at the top. (Let’s get this out of the way at the beginning: What physicists really care about is “mass” and the distinction between mass and weight is near and dear to our hearts. But at the elementary school level it’s not worth worrying about, and “weight” is more familiar, so I’ll stick with that.) In fact when we start learning physics we treat objects as if weight were their only property, and it’s remarkable how much you can understand about the world without worrying about any properties of an object other than its weight.

It is an object’s weight, above all, that determines how it will move in response to forces — the heavier an object is, the less effect a force will have on it. The weight also determines how strongly it is attracted by, and attracts, other objects through the force of gravity. Finally, at the most basic level, the weight tells you how much stuff is in the object: how many protons and neutrons (and electrons, though they don’t weigh much) make it up. A pound of lead and a pound of chicken feathers contain essentially the same number of protons and neutrons — put together in very different ways, to be sure, but still the same number.

Weight is also important because it’s a conserved quantity — that is, the total weight of the matter in a closed system doesn’t change, whatever dramatic or even violent physical and chemical processes may take place: explosions, collisions, melting, freezing — the total weight doesn’t change. (Okay, that’s not quite true; the famous equation E = mc2 tells us that it’s possible to change weight into energy and vice versa, but that becomes important only in some rare and extreme situations.) Scientists treat conserved quantities, like weight and electric charge, with special reverence. From a practical standpoint, the fact that they remain constant while other things are changing gives us a useful handle for analyzing complicated phenomena. And on a more fundamental level, the fact that they are conserved says something deep about the structure of the universe, though it’s not always easy to say exactly what.

Finally, scientists care about weight because we don’t fully understand it. We don’t know where it comes from, and there is great excitement in the world of physics right now, because we hope that experiments at a new particle accelerator in Europe will confirm the most popular theory for the origins of weight by discovering something called the “Higgs particle”. Sometimes the most basic things in science are the most mysterious.

—Roger Tobin