Gravity 1/3/20

A Science discussion of Gravity by Richard Bleil

Let’s talk about a heavy subject, namely, gravity.

Okay, fine, gravity in and of itself is not heavy (as far as we currently understand although recent discoveries suggest it exists as a wave which, thanks to deBroglie, would imply it also has a particle component). But it is gravity that gives objects weight, and maybe that’s a good place to start.

People often use the terms “mass” and “weight” interchangeably while they are actually different albeit related. See, mass is simply defined as “quantity of matter”. The greater the mass, the more of that matter you have. Ten grams of air has just as much matter as ten grams of lead, the only difference being that the ten grams of lead is in a much smaller volume. Weight, on the other hand, is the force that matter is exerting on another object. Weight is mass times gravity. This is why the same mass will have different weights on earth (with a higher gravitational constant) than on the moon. It’s also why the weight of that same mass will be different at sea level than on the top of a mountain since the further away from the earth’s core one is the smaller the gravitational pull. It’s also why that same mass will weight less at the equator than it will on the poles, since centrifugal force is working in the opposite direction (trying to throw the object off of the earth) than gravitational force because of the direction of motion due to earth’s rotation.

As you can see, weight varies even when mass remains the same. How much lead you have is the same regardless of its weight, and this is measured as mass. This is why scientists work with mass rather than weight.

Today, I was reading an article of the principal tenets of “flat-earthers”. I truly hope that people who believe that the earth is flat is basically a joke, something said tongue-in-cheek to throw in the face of society as a whole. I realize that there may be some people who truly believe that the earth is flat, but with the upwards trend of those saying they believe it I’m hoping it’s more a form of rebellion than an actual belief. Anyway, one of the central tenets of the “flat earth” society is that gravity is a misconception. The reality is that everything is really just falling, which is why gravity appears to be real.

Okay, let’s go through a couple of problems with this hypothesis. First of all, in the vastness of space, “up” and “down” are relative. If everything is falling, to what is it falling? And why is that direction, in particular, “down”?

The bigger problem is the question of why we can’t all fly? See, if we are falling, and the earth is falling, then what is it that makes us fall faster than the earth? And if we’re falling at the same speed as the earth, well, we wouldn’t be standing on it. It was Isaac Newton who proposed that all objects fall with the same acceleration barring air resistance. Drop a hammer and a feather and of course the hammer will fall faster because the feather, by its design, has greater friction with the air which birds use to give them lift during flight. Repeat the same experiment on the moon, as astronaut David Scott did during the Apollo 15 mission, and you’ll find that an odd-looking phenomenon happens. The hammer and the feather do, indeed, fall at the same acceleration (most people say “velocity” rather than “acceleration”, but free-fall is actually an acceleration, so the velocity is not at all constant).

Understanding that, let’s talk for a moment about the reach of gravity. People often speak of the “gravity free” nature of space. In fact, gravity is attraction of any mass in the universe to every other mass in the universe, an attraction that has, quite literally, no known distance limitation. Everything in the universe is attracting you, but, it does so at a “squared” effect. In other words, double the distance, and the gravitational force is one fourth the value. Still, there is no distance where this force becomes zero, although it becomes infinitesimally small (negligibly small) pretty quickly. This phenomenon has been known for a while, so in modern times people speak of “micro-gravity” rather than “no gravity”, but even this is a misconception. The moon is a painfully small body compared to the earth. Earth has more than eighty times greater mass than the moon, so the gravity on the moon is already one eightieth that of earth were you to stand on it. What’s more, the moon is over two hundred thousand miles away. The earth has a radius of about four thousand miles, so the distance of the moon is fifty times the radius of the earth. Gravitationally, this means that, if the bodies were of equal mass, the moon’s gravity would be 1/2,500 th that of the earth. Take the mass difference into account, and you’re feeling the effect of the earth’s gravity two hundred thousand times more strongly than that of the moon, and yet the pull of the moon’s gravitational force gives rise to effects like the tides.

The reason astronauts feel (and it’s only a feeling) “weightless” is because they are actually in free fall. That is, their acceleration towards the earth (which is a delicate dance when they’re in orbit, where they are falling towards the earth at exactly the same rate that they are being flung out into space, so neither is happening) is exactly equal to the acceleration of their space ship and its contents. You’ve experience free fall, or weightlessness, yourself to a lesser effect. Any time you’re in an elevator going down, you feel “lighter”. Your acceleration towards the earth is still faster than that of the elevator, but because the controlled descent of the elevator is in the same direction as the gravitational force pulling you down, you have partial weightlessness. In roller coasters and other amusement park rides, they play with this feeling frequently, including one ride that literally takes the riders to a height and simply drops them. This is as close to free fall “weightlessness” as you can get. It also happens when a plane suddenly drops due to and air pocket. The plane cannot fall faster than the acceleration due to gravity, but the closer to free fall it gets (and the farther), the closer the feeling of weightlessness (and the longer).

Okay, I’m done. I’m dropping this subject. I know it was a heavy topic. There was a lot to chew on. And whatever additional gravitational puns you would like to throw in here.


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