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Chapter_6

# Chapter_6 - Chapter 6 Force and Motion II In this chapter...

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Chapter 6 Force and Motion II In this chapter we will cover the following topics: frictional force (static and kinetic friction) drag force and terminal speed revisit uniform circular motion by using the concept of centripetal force and Newton’s second law to describe the motion. (6-1)

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(6-2) no push, no friction a little push, no movement static friction ! F net,x =F- f s =0 push harder (F increases), still no movement F- f s =0 ( f s increases) static friction force kinetic friction force S f k f if F> f smax movement starts kinetic friction F net,x =F- f k =ma movement with constant velocity F net,x =F- f k =0 x
S Smax k Smax k Smax f = f = F (F < f ) f = f = const (F > f ) f < f Static Friction vs. Kinetic Friction F F N mg

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F F N mg The frictional force is acting between two dry unlubricated surfaces in cont act Properties of friction : If the two surfaces do not slide relative to each other, then the static frictional force = ( ) The magnitude of the static friction v if a=0, a ries fro the n = m 0 - Property 1. Property 2. s s net F f F f ,max 0 to (the constant is the . Once the crate starts to move, the frictional force is kinetic friction. Its magnitude is (independ = = Property 3. s s N k k N s k f F f F f μ μ μ coefficient of static friction) ,max is the . Note: The static and kinetic friction act parallel to the surfaces in contact The direction th ent of F e direction of mot o ) i ( < < Note : k s k k s f f μ μ μ opposes coefficient of kinetic friction) n (for kinetic friction) or intended motion (for static friction) (6-3)
Α t what angle θ can the coin stay at rest? s s smax s s s 1 max s N W cos 0 f Wsin 0 f f N Wsin W cos tan tan - - θ = - θ = = μ θ ≤ μ θ θ ≤ μ θ = μ θ x y

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6.3.3. A crate of mass m is at rest on a horizontal frictionless surface. Another identical crate is placed on top of it. Assuming that there is no slipping of the top crate as a horizontal force is applied to the bottom crate, determine an expression for the magnitude of the static frictional force acting on the top crate. a) b) c) d) e) F F f = mg F f - = 2 2 mg F f - = 2 mg f = 2 F f = F
6.3.3. A crate of mass m is at rest on a horizontal frictionless surface. Another identical crate is placed on top of it. Assuming that there is no slipping of the top crate as a horizontal force is applied to the bottom crate, determine an expression for the magnitude of the static frictional force acting on the top crate. a) b) c) d) e) F F f = mg F f - = 2 2 mg F f - = 2 mg f = 2 F f = 2 = F ma = s f ma s f F x - s f / 2 = s f F max = s s s f f mg μ

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Example 1 28. Figure 6-35 shows three crates being pushed over a concrete floor by a horizontal force of magnitude 440 N. The masses of the crates are , , and . The coefficient of kinetic friction between the floor and each of the crates is 0.700. (a) What is the magnitude F 32 of the force on crate 3 from crate 2? (b) If the crates then slide onto a polished floor, where the coefficient of kinetic friction is less than 0.700, is magnitude F 32 more than, less than, or the same as it was when the coefficient was 0.700?
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