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Lecture_7

# Lecture_7 - 4.9 Frictional Forces Weve been discussing...

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4.9 Frictional Forces We’ve been discussing forces on objects who have surfaces in contact. The Normal Force is perpendicular to the surface. Normal Force is perpendicular to the surface. There is another force due to surfaces in contact in which the force is parallel to the surface. This is call the Frictional Force . W ill ft j t f t thi f F i ti We will often just refer to this force as Friction . Friction comes in two types: Static Friction Kinetic Friction Consider the situation of a block sitting on a table: The Frictional Force tends to oppose the direction of motion, i.e. it tries to slow the object down. But not always!!! If I want to slide the block across the table, then I have to overcome a frictional force just to get it to move (static friction) (static friction ). And then I have to overcome another frictional force to k it i Th t i th ki ti f i ti f i ti f keep it moving. That is the kinetic friction , or friction of motion.

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F f k Let’s say I want to try and pull the block to the right. As I start to pull with some force F , the block doesn’t move because of the static frictional force ( f s ). Notice that f s is parallel to the surface of contact, and it opposes the direction that I’m trying to pull the block. So I keep making my pulling force F bigger and bigger. f s keeps getting bigger and bigger too, canceling out my pulling force until f s reaches its maximum value.
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Lecture_7 - 4.9 Frictional Forces Weve been discussing...

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