 We're going to look at two kinds of friction, static and kinetic friction. Now, here's an example of kinetic friction. If I push this brick, it slides across the table, obviously, and comes to a stop. Something is bringing it to a stop. We take that to be a friction force. Now, since the brick is moving, that's called sliding friction, or another word for it is kinetic friction. Now, the characteristic of kinetic friction force is that the force always acts opposite the direction the object is moving. So the object, the brick moves this way, the kinetic friction force is that way. All right, let's take a look at another kind of friction force. This one has an example of this one is the force between the tires of a car and the road. Let's get this car going. Now, the force that pushes the car along the road, forward along the road, is a friction force. Now, that may seem like a strange thing to say. After all, isn't it the engine that moves the car? Well, you certainly need the engine in order to move the tires, but you need the road in order to move the car. To see why that's true, suppose I turn on the engine. But with no road, it doesn't go anywhere. The tires simply spin. The road provides the friction. What's actually happening when the tires are in contact is that the tires are pushing backward on the road, and the road is pushing forward on the tires. This is an action-reaction pair of forces that acts according to Newton's third law. So tires push back on the road, road pushes forward on the car, or on the tires. And that push force is a friction force. And in this case, we call it a static friction force. We use the word static because static implies no motion. Now, it's not that the car isn't moving, it certainly is. But the wheels are not slipping along the roadbed, at least not in normal operation. Now, here's another example of static friction. When we put an object on this brick, if I tilt the brick a little bit, the object stays there. Well, what keeps it there? It's a static friction force. There's no motion between the bar and the brick. Now, if I tilt it enough, the bar begins to move. And so the static friction force became a kinetic friction force at that particular point. One more example of static friction force has to do with the force required for us to walk. In order to walk across the floor, the floor has to push you forward. It's a static friction force. What's happening is that your shoes are pushing backward on the floor, and the floor is pushing forward on you. Again, if that seems strange, just imagine that there were no friction between you and the floor. Maybe it's a icy surface, or maybe you're walking on ball bearings. In that case, you simply slip and fall. You don't go anywhere. So you can try as hard as you want, but you're not going to move. So you've got to have that static friction force between your feet and the ground in order to move. So static friction is necessary for human locomotion, at least the kind that we normally do.