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Kinetic and Static Friction Forces

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We are going to look at two kinds of friction, static and kinetic friction. Here is an example of kinetic friction. If I push this brick, it slides across the table and comes to a stop. Something is bringing it to a stop, we take this to be friction force. Since the brick is moving, that's called sliding friction, or another name for it is kinetic friction. Now, a characteristic of kinetic friction force is the force always acts in the opposite direction that the object is moving. So if the brick moves this way, the force acts that way. Let's take a look at another kind of friction force. 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 is a friction force. Now that may be a strange thing to say, after all isn't it the engine that moves the car forward? Well, you certainly need the engine to move the tires, but you need the road to move the car. To see that that is true, suppose I turn on the engine, but with no road it goes nowhere. The road provides the friction. What is actually happening when the tires are in contact is the tires push back on the road, and the road pushes forward on the tires. This is an action-reaction pair of forces, that acts according to Newton's third law. So the tires push back on the road, and the road pushes forward on the tires. That push force is a friction force. In this case, it is a static friction force. We use the word static, because static implies no motion. Now its not that the car is not moving, but it's the fact that the tires are not slipping, at least in normal operation

Another example of static friction, let me put on object on this brick. If I tilt the brick, the object does not move. Well that keeps it there? It is a static friction force. There is 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 becomes a kinetic friction force at that moment.

One more example of static friction force is the force required for us to walk. In order to walk, the floor has to push you forward. What's happening is your shoes are pushing backwards and the floor is pushing forward on you. Again, if that seems strange, imagine that there were no friction between you and the floor, like if it were an icy surface or if you were walking on ball bearings. In that case, you simply slip and fall, you can try as hard as you want, but you will not move. You need that static friction force to move. Static friction is necessary for human locomotion.


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