 Now we can consider the normal force. So our first question is, what do we mean by normal force? Let's start with one word at a time. Force, we've already talked about, is a push or pull on an object. So then we get to normal. Now what we don't mean by this is it's not an ordinary force. While they happen all the time, that's not the usage of the word normal we mean. Instead we're talking about a mathematical definition of normal that refers to the perpendicular. So for example, if I have a surface, my normal force is always going to be at a 90 degree angle perpendicular straight out from that surface. And I use the f with a sub n as my symbol for it, both capital F and capital N. So here's some examples. Again, if I have a horizontal surface, my normal force might be straight up from that surface. If I have a vertical surface, my perpendicular is going to be, say, out to the left. And if I have a sloped surface, my normal force is still going to be perpendicular, but that means it's going to be off at an angle. Now when do you have the normal force? Anytime you have solid surfaces in contact with each other. The normal force is the force that prevents those solid objects from going through each other. Now from Newton's third law, we know that forces come in pairs. So let's understand that in terms of a normal force. The ground pushes up on the block with a normal force, but the block also pushes down on the ground. If I push on the wall to the right, the wall pushes back on me to the left. These are force pairs. I push on it, it pushes back on me. In both cases, I can't push through the wall and the wall doesn't push through me. Now if I've got multiple surfaces, like I have in this example, I have to consider a normal force for each one of the points of contact. So for example, up here at my hands, I have a normal force where I'm pushing on the wall and the wall is pushing back on my hands. In this case, my front foot is going to have a second normal force, where my foot pushes down on the floor and the floor pushes up on that front foot. And in the back foot, we have a third normal force. And again, it's a force pair where my foot pushes down on the floor and the floor pushes back up on my foot. If I have multiple objects, I need to consider the surfaces for each one of those objects. So here I've got two boxes sitting on top of each other, sitting on the floor. Well, there's a surface between the two boxes, and so the green box pushes up on the blue box, but the blue box is also pushing down on the green box. And there's a surface contact between the green box and the floor so that the floor pushes up on the green box and the green box pushes down on the floor. When I go to do force diagrams, remember that I want to treat each object separately. So my blue box has a normal force upwards due to the green box. And my green box has two normal forces, one due to the floor and one due to the box sitting on top of it. Now don't forget both of those boxes also have a force of gravity assuming they're sitting here on Earth and not out in space. And so your full force diagram would have to take into account not just the normal forces, but any other forces that are on there as well. So that's your introduction to the normal force. Thank you.