Intro to Normal Forces | Part 1 - Nerdstudy Physics

Introduction to the concept of Normal Forces! Let's go into detail about how this concept works and if we can apply Newton's Law to this idea or not.
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uppose we place a book on a table. We all know what happens, right? Once the book settles into position, nothing happens at all! That’s pretty obvious, but I want to draw your attention to one of the things that doesn’t happen, namely falling down through the table toward the ground. Why doesn’t that happen? After all, gravity is pulling the book downward, right? Well, clearly, the table exerts an upward force which prevents the book from falling downward. It counteracts the force of gravity by exerting a force on the book which is exactly enough to prevent it from moving downward.
Now that sounds familiar, doesn’t it? There’s a force acting downward, namely gravity, and there’s a force that’s exactly the same in magnitude but in the opposite direction. They cancel out exactly, and the result is that the book doesn’t move at all. Hmm … Doesn’t that sound like Newton’s Third Law? That’s the one that says, “For every action there’s an equal but opposite reaction”. It seems like this would be the reason for why the forces cancel, but is it? Do you think that the force of gravity and the force of the table cancel each other out due to Newton’s Third Law?
Well, the answer is actually no, Newton’s Third Law doesn’t guarantee that gravity and the force of the table will cancel each other out. Earlier, we quoted a common paraphrase of Newton’s Third Law by saying that for every action, there’s an equal but opposite reaction. But we should proceed more carefully. What does Newton’s Third Law really say? It says that when one object exerts a force on a second object, the second object exerts an equal but opposite force on the first object. Once we look at this full statement of the Third Law, we notice that the two forces mentioned in the Law actually act on two different object.
Alright, with that in mind, let’s look again at our physical situation. There’s a book lying on a table, and two forces are acting on the book. First, gravity is pulling the book downward. And second, a force from the table is pushing it upward. These two forces may be opposite but equal, however there’s one important fact that prevents Newton’s Third Law from applying here. And what would that fact be? Well, the two forces are acting on the same object, namely the book! But the pair of forces in Newton’s Third Law act on two different objects. Therefore Newton’s Third Law is not a good explanation for why the book doesn’t fall downward through the table.
Okay then, what is the explanation? Actually, the true explanation is quite complicated and we would need to invoke advanced topics like quantum mechanics. At this point we can only give you a very rough idea right now. Basically, there are two factors involved. The first one is that protons and electrons, which are two types of particles that make up everyday things like books and tables, are electrically charged. Therefore, they attract or repel each other, sort of the same way magnets do. The other factor is that, for the particles that make up everyday things, it’s impossible to make two of them occupy exactly the same spot. Based on these two factors, physicists can give an explanation of why books don’t fall through tables.
It would be terrible, though, if we had to talk about particles and electric charges every time we have to perform a calculation. I mean, books lying on tables are a very common sight. Even though we can’t fully understand why books don’t fall through tables, can we at least describe what happens in terms of forces and such? Well, we certainly can.
Looking at the book on a table again, we already know what happens in a general sense. No matter what force causes a book to press against a table, the table will exert a force to cancel it out. This cancelling force will be exactly enough to prevent the book from moving through the table. And that’s basically all we need to know! Somehow, this complicated interaction between microscopic particles and material combine to produce this phenomenon: no matter how hard a book presses on a table, the table produces a force that’s exactly enough to counteract it and prevent the book from passing through the table. Obviously, this isn’t true if the book’s so heavy that it’ll break the table, but we’re going to ignore that possibility in this lesson.