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Understanding Friction: Definition, Causes, and Effects

Learn about friction, the reaction force between surfaces in contact that affects motion, and its causes including surface nature and normal force. Discover static and kinetic friction, coefficient of friction, and more.

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Understanding Friction: Definition, Causes, and Effects

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  1. Friction

  2. Definition and Cause • A reaction force present when one object is in contact with another and moves (or tries to) across their mutual surface • Due only to two factors: • The nature of the surfaces in contact (smooth, rough, etc) • The amount of normal force exerted by each surface on the other (Fn) • Surfaces: even the smoothest surfaces have imperfections (ridges, depressions) at the microscopic scale From http://www.pxleyes.com/blog/2010/03/5-things-you-need-to-know-about-raytracing/ • When these surfaces move across each other, the ridges and depressions bind and release, generating a force in the opposite direction to motion

  3. Causes (cont.) • Given the previous picture, explain why an object in contact with paper has more friction than the same object in contact with aluminum. • Explain the statement: the amount of friction between two surfaces is different than the amount of friction between any two other surfaces • Changing the normal force also changes the amount of friction: FN Ff Fapp FN Fg 2 x mass = 2 x FN = 2x Ff Fapp Ff Fg

  4. Example Fappx • The Fappshouldbe broken into its components: • Fnet= 0 (uniform motion) • Ff = -Fappx • Fappx= 150 (cos 15) =144N Ff = -1.4 x 102 N = • What is the force of friction if the crate is sliding at a constant speed when being pushed at an angle of 15 (down) with a force of 150N? 15 Fappy 150N FN Ff Fappx Fapp Fg+ Fappy

  5. Example 2 Fappx • Look at the vertical components now • Fnet= 0 (uniform motion) • FN = -(Fg+ Fappy) • FN = -(mg + -150sin15) =144N=-[(58kg)(-9.81m/s2)–150sin15]= 607N = 6.1x102 N • If the crate is 58kg, what normal force is the ground applying to it? 15 Fappy 150N FN Ff Fappx Fapp Fg+ Fappy

  6. Static and Kinetic Friction • Static friction occurs between two surfaces before motion begins – tries to prevent motion • Strongest Frictional Force • Kinetic Friction occurs between two surfaces after an objet is in motion…tries to slow it down • Weaker than static friction • Explain Why?

  7. Coefficient of Friction • Since the amount of friction is different for every 2 surfaces, and proportional to Fn, we can define an equation for Ff • Ff = Fn , where  (pronounced mue) is called the coefficient of friction ( •  is determined empirically (experimentally) for every 2 surfaces in contact • Every two surfaces have a static  and a kinetic  • A simple set-up to measure : • Using Fnet= Fapp+ Ff = 0 if the block is not moving (static) or moving at constant speed (kinetic) then:Ff = -Fapp(measured by scale) • Ff = Fn so  = Ff/ Fn • Fn = -mg (in this case) •  = Ff/(mg) [note: drop all‘-’ signs…just interested in number not direction] Picture Reference

  8. Coefficient of Friction example • A 75.0kg sled needs 250N of force to get pushed into motion. What is the coefficient of static friction between the runners and the snow? • Since Fnet= 0 (looking at the point of first motion) • Ff = Fapp= -250N • Ff = Fn so  = Ff/ Fn • Fn = -mg (in this case) •  = Ff/ (mg)= 250N/[(75.0kg)(9.81m/s2)]= 0.334 FN Ff Fapp Fg

  9. Coefficient of Friction example 2 • The coefficient of kinetic friction for the same sled is 0.25. Calculate how much force it would take to accelerate it at 3.2 m/s2 after it was already in motion. • Fnet= Fapp+ Ff = ma • Fapp= ma – Ff • Ff = -Fn (direction is important here!) • Fapp= ma –(-Fn) • Fn= -mg • Fapp= ma –[- (-mg)]= (75.0kg)(3.2m/s2)+[0.25(-75.0kg)(-9.81m/s2)] = 424N = 4.2x102N FN Ff Fapp Fg

  10. Assignment • Read pg 172-173, 182-183 • Do pg 172 pracprob 1 • Do pg 185 pp 1-2 • Do pg 188 pp1 • Work on assignment (can do coefficient of friction questions)

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