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Chapter 6

# Chapter 6 - Anowncement I email HITT scores last Friday...

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Anowncement I email HITT scores last Friday Exam #1: Wednesday, June 3, in class Exam #1 covers ch.1-ch.6.

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Chapter 6 Force and Motion ||
Friction When: when in contact with a rough surface. Slippery surface - no friction! Application point: along the surface of contact. Direction: opposite to the attempted slide. Magnitude: see next chapter r f r f r υ r υ r f

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Friction, detail (1) Body at rest Body at rest & pushed 0 , = x net F x y r F N r F g r F net = m r a 0 , = y net F F N - F g = 0 0 = f x y r F g r F N r f F r r F net = m r a 0 , = x net F 0 , = y net F F N - F g = 0 F f = 0 = - f F
Friction, detail (2) Body moving with const v Body moving with const a 0 , = x net F x y r F N r F g 0 = = a m F net r r 0 , = y net F F N - F g = 0 F f = x y r F g r F N r f F r r F net = m r a x x net ma F = , 0 , = y net F F N - F g = 0 x ma F f - = x ma f F = - F r

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We can explore the basic properties of friction by analyzing the following experiment based on our every day experience. We have a heavy crate resting on the floor. We push the crat Friction: e to the left (frame b) but the crate does not move. We push harder (frame c) and harder (frame d) and the crate still does not move. Finally we push with all our strength and the crate moves (frame e). The free body diagrams for frames a-e show the existence of a new force which balances the force with which we push the crate. This force is called static frictional fo the . As we increase , rce s s f F F f r r also increases and the crate remains at rest. When reaches a certain limit the crate "breaks away" and accelerates to the left. Once the crate starts moving the force opposing its motion is called th F e . . Thus if we wish the crate to move with constant speed we must decrease so that it balances (frame f). In kinetic frictional force frame (g) we plot versus time k k s k f f f F f f t < r (6-2)
F F N mg The frictional force is acting between two dry unlubrica Properties of ted surfaces i frictio n cont n: act If the two surfaces do not move with respect to each other, then the static frictional f Property 1.

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