This series of lessons is designed to help you learn, or review, the fundamentals of physics. This part focuses on static and kinetic friction.
So what's friction and what is it good for?
Friction is the force that opposes motion. For example, try sliding a heavy book across the floor. Soon enough, it'll stop. That's because the surface of the book and the floor interact in a way that results in the book slowing down- there is a force acting on the book in the backwards direction.
Friction is a good thing. Without friction, we wouldn't be able to walk without falling down, because no force would stop our feet from moving forward. In fact, we would never be able to stop at all. A lot of machines are based on friction and the heat it creates (kinetic energy of motion converts to heat as a result of friction). But friction has some bad sides to it. Friction prevents us from going as fast as we may like. Friction also causes wear and tear of various materials.
So how does friction work?
We know things are made of tiny particles, and these particles have forced of attraction. When a book slides across the floor, the particles (atoms and electrons) of the book and of the floor attract, which causes the book to slow down.
There are ways to reduce friction- some substances cause less friction by nature, since they have less particles or less electrons in their molecules. These can be used as lubricants to reduce friction. As well, you'll note that things that roll have less friction than things that slide due to the mechanics of that motion.
Note that we're only concerned today with dry friction. Fluid friction is a slightly different matter, but in simple physics it's usually ignored (air friction is a great example) since it's negligibly small.
Still, there are 2 principle types of friction- static and kinetic.
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Static
friction- don't move
Static friction is the force that resists motion when there is no motion. This is a bit hard to imagine… so here are two examples: Imagine a low slope. Now put an object on it. The object won't slide down when the slope is fairly low, but when you look at this FBD (which we've seen earlier), it seems like the net force should push the object down: |
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However, static friction exists, and the picture should really look like this: Now, for the second example, imagine a big, heavy box. If you push it with small force, it won't move. How come? Static friction again. So you see, static friction prevents motion from occurring. |
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Kinetic motion- move, but less
Kinetic motion resists motion when things are already moving. You can look at the sliding book example to see how it works.
There is a
useful equation for friction: , where
mu (usually denoted by the Greek letter Myu which I have no idea
how to draw here) represents the coefficient of friction, which is
really the ratio between the kinetic friction and normal force
acting on an object. In horizontal cases , since normal force
equals gravity, we can simplify this to
. If you're given a coefficient of friction, you
can calculate the force of friction, and vice versa.
Friction is pretty cool, isn't it?
Next time, we'll combine all the different forces and look at force problems.
