Did you get the pun in the title? Matter? Gravity? It’ll make more sense after you read the article, and I’m sure you’ll think it’s hilarious.
Our previous article was mostly about scale, trying to give everyone a sense of the size and scope of the universe, from the very small, like atoms and subatomic particles, to the very large, like the size of the universe. This opens the door to think about matter, or as most people call it—“stuff”. The dictionary definition of matter is anything that takes up space and has mass. The “taking up space” part of that definition is easy enough to understand. Everything has to be somewhere. The “has mass” part also is straightforward, if you equate mass to weight, which is okay for everyday use, but a bit of an oversimplification. Mass is something that all matter has, and it doesn’t change. It’s a measure of how much “stuff” in in something. The mass of a baseball on the Earth is the same as the mass of a baseball floating in space out on the other side of Pluto somewhere, because a baseball is still a baseball, no matter where it is. Weight, however, can change, depending on where you are. For instance, if you weigh 200 pounds in Poughkeepsie, you would only weigh about 33 pounds on the Moon. Even though your mass never changes, your weight changes because the force of gravity on the Moon is roughly 1/6 that of Earth. The Moon exerts less gravitational force than the Earth because the Moon has less mass. The Moon has only about 1/6 the mass of the Earth, so it exerts 1/6 of the gravitational force on you that the Earth does. One cool thing to understand about gravity is that it is a two-way street. Gravity is a measure of the force between any two objects that have mass. How large the force is depends on the masses of the objects involved. The Earth exerts a gravitational force on you, but you also exert an equal gravitational force on the Earth, because you have mass, too. If you jump in the air, the gravity generated by your mass and the mass of the Earth will pull you back down. On the other hand, the gravity generated by your mass will also pull the Earth up to meet you, just as hard as the Earth is pulling you down. The Earth is a little more massive than you are, so you will move toward the Earth a lot more than the Earth will move toward you, but it will move just the tiniest bit toward you. The difference in how much you move toward the Earth relative to how much the Earth moves toward you is because of a property of mass, called inertia. All objects with mass have inertia. The more mass an object has, the harder it is to start it moving, stop it or make it change directions. You’ve heard about inertia. You experience it every day and adjust to it constantly. When you drive, for instance, you are constantly making adjustments to the motion of your car to make it go where you want it to. If you don’t, it will just keep going in a straight line until it hits something or stops because of the friction in all the moving parts of the car. If someone throws you a ball, you can look at where the ball is and how fast it’s moving, and you can put your hand in the right place to catch it, because you have instantly analyzed its motion and how gravity and inertia will act on it. Are you surprised to learn that you already knew all this stuff? You already know what inertia is. You know what gravity is. You know how to calculate (in your head, no less!) the effect of gravity and inertia on the motion of objects like a ball. I’m guessing that you might not have known that you already knew that. I’m almost certain that you were unaware that you also already know Newton’s First Law of Motion. It says (more or less) that any object will remain motionless unless some force (like gravity, the force generated by your arm muscles or the engine of your car, etc.) acts on it to make it move. Once moving, the object will continue to move at a constant velocity in a straight line forever, until another force (friction, gravity, etc.) causes it to slow down, speed up or turn. You already know all that. You just didn’t know it was Newton’s First Law. Like I told you in the first article in this series–you know a lot more science that you think you do.
We’ve established that you already know a lot about mass, gravity, and inertia. Here’s a question for you that I’ll bet you will get wrong: Is the force of gravity a powerful force or a weak force? Think about it. I’ll wait.
So, gravity is strong, you say? Of course it is! It holds everything down. The gravity between the Sun and the planets keeps the planets from spinning off into space, for goodness sake! Of course gravity is a powerful force. Silly question, right? Nope. Gravity is actually an incredibly weak force. Of the four fundamental forces in the universe (gravity, electromagnetism, and the strong and weak nuclear forces [forces that work to hold the parts of atoms together]) gravity is actually, by far, the weakest, by many orders of magnitude. If you want to test this fact, do what scientists always do—conduct an experiment. Reach out and try pick something up (experiments don’t have to be complicated). It doesn’t matter what it is—a pencil, a rock, a doughnut, whatever. Were you able to pick it up? You just overcame the force of gravity generated by the entire Earth. Your puny little arm can easily generate more force than the gravity produced by the mass of the Earth. So if gravity is so weak, why doesn’t the Moon float away? Why don’t the planets spin out into space? Yes, gravity is a very weak force, but if the objects are massive enough (like stars and planets), it’s enough to hold the universe together. Pretty cool, huh?