Gravity
I. The popular conception of "gravity" is a reification, which we were taught as an explanation for a phenomena that it merely labels. This is sad, since the more that people are taught to think with reifications, the more we will tend to accept empty explanations for very real problems.
A. When asked why things fall to the earth, people often provide the reification we were taught in grade school: gravity. Gravity makes things fall to the earth. Yet, when asked what gravity is, few of us can say more than, "Gravity is what makes things fall to the earth." This is circular—we haven't answered the question. Those who remember high school physics regrettably don't fare any better. They might say, "Gravity is a force that attracts things together. It's related to an object's mass: the more mass an object has, the more gravity it's exerting in every direction. We generally notice gravity only when an object—like a moon or a planet—has a lot of mass. Then, the object has a lot of gravity, too. When you put an object with less mass (and hence less gravity)—say, a frying pan—near an object with a lot of mass (and hence a lot of gravity)—say, the earth—the earth's gravity moves the frying pan toward it, while the frying pan doesn't have much effect on the earth. Thus, the frying pan appears to fall—because of gravity."
B. This explanation,
too, breaks down.
Q: What's mass?
A: The amount of matter in an object.
Q: What's matter?
A: Anything that has mass and takes up space.
Q: You say gravity is a force. Can you measure it?
A: No.
Q; How do you know it's there?
A: Because it makes things fall.
Q: So, things fall because a force that makes them fall…
well… makes them fall. Moreover, everything has this force
because it has mass, which is a measure of "how much of
anything that has mass and takes up space" it really has.
Did that explain anything to you?
A: Not really.
Q: Me, neither. Would you like to hear my explanation?
II. Here is a better attempt at answering, "Why do things fall to the earth?"
A. We describe objects with certain words, called "attributes."
1. One object may be bigger or smaller than another; we call that the attribute of size.
2. One object may be heavier or lighter than another; we call this the attribute of weight.
3. Other things being equal, a heavier object exerts more pressure downward than a lighter object—which is why I'd rather have a hamster sit on me than an elephant. When two objects are the same size but one is heavier, we say that the heavier one is denser than the lighter one. When we say that a marble is denser than a styrofoam ball, we mean that there is more "stuff" to the marble than there is to the styrofoam ball; we're referring to the attribute of density.
4. Some objects break when we pull, push, rub, bend, or twist them; others change shape and never return to their original shape; still others change shape and return to their original shape. We call objects that break when we pull, push, rub, bend, or twist them "inelastic"; we call objects that change shape and never return to their original shape "plastic"; we call objects that change shape and return to their original shape "elastic." When we talk about how an object responds to pulling, pushing, rubbing, bending, or twisting, we're referring to the attribute of elasticity.
B. Now, other things being equal, objects less dense than what surrounds them rise. A cork is less dense than liquid water, so a cork rises in liquid water—then stops at the top of the water because a cork is more dense than air. A helium balloon is less dense than air, so a helium balloon rises in the air. Things that are less dense than the liquid or gas they're in rise; I'll call this the "density exception" to the rule that things tend to fall.
C. If I'm standing on solid ground, I can lift a 40 lb. weight because my muscles can press upward against the weight and downward against the earth hard enough to lift it, while my fascia, bones, muscles, and skin hold me together. (If I were sinking in quicksand, I may not be able to lift that 40 lb. weight, since I couldn't press downward against solid earth. I don't know because I've never tried this. I'm not likely to ever find out, either, because if I were sinking in quicksand and happened to have a 40 lb. weight, I probably focus more on getting out of the quicksand than on trying this experiment.) I can lift a 40 lb. weight, but I can't lift a 400 lb. weight: I'm not strong enough to push downward against the earth and upward against a weight with more than 400 lb. of pressure. I can, though, not only lift a basketball; I can throw it: I can press up on the ball and down on the earth very quickly to make the ball rise, leave my hand, and fly through the air. This happens even though the ball is more dense than the air around it. (The ball will come back down after a while, too, since I can't keep pressing from underneath it once it's left my hand, and it is more dense than the air around it.) Other things being equal, things that have more pressure upward from beneath them than their weight will rise; I'll call this the "lift" exception to the rule that things tend to fall.
D. Some pressure is in every direction, not just up and down. Think of the pressure inside a basketball: when someone pumps air into the basketball, it gets hard because there's more air per cubic inch inside the ball than outside. The air inside the ball is applying a lot more pressure outward, in every direction, than the air outside is applying inward, from every direction. When air moves past an airplane wing really quickly—for reasons I won't get into here—the air pressure underneath the wing, in every direction, is more than the pressure above it, from every direction. If there is enough pressure underneath the wing to overcome the weight of the plane, the plane will rise—even though the plane, like the basketball, is more dense than the air around it. So, differences in pressure can make things rise; I'll call this the "pressure exception" to the rule that things tend to fall.
E. There are two final exceptions to the rule that objects tend to fall.
1. When an object isn't moving in relationship to its surroundings, it's not going to fall unless something moves it; I'll call this the "stationary exception."
2. I'll call the final exception the "elasticity exception": sometimes, when an elastic material collapses quickly, it expands quickly, too. Rubber balls that bounce (called Super Balls) are made of a material that quickly collapses and quickly expands. After a Super Ball hits the ground, it quickly collapses a bit, then while still on the ground, quickly expands a bit, too. That expansion pushes the ball off the ground, effectively lifting it. (Notice the similarities between the elasticity exception and the lift exception.)
F. If none of these five exceptions apply, an object will fall to the earth. I don't quite know why, but I know it will happen consistently.
III. Now that's an explanation! In it, the speaker ultimately pleads ignorance as to why things fall to the earth, but (s)he discusses relationships that an audience, to the extent it understands them, can use to make predictions about future occurrences of things falling—or not falling. What's sad is that the above response—despite its quality explanation—may actually receive a lower grade in junior high or high school science class than a response which offers the expected reification of "gravity."