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Friction

Friction. Friction: F f ● the force that opposes motion when two surfaces are in contact. Kinetic Friction : F k ● sliding friction when two surfaces are moving relative to each other. Static Friction : F s ● the force that resists the start of motion . Example 1: Applying force

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Friction

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  1. Friction

  2. Friction: Ff● the force that opposes motion when two surfaces are in contact Kinetic Friction: Fk ● sliding friction when two surfaces are moving relative to each other Static Friction: Fs● the force that resists the start of motion

  3. Example 1: Applying force -fridge not yet moving -static friction balances applied force +y +x FN FBD: Fapp Fs Force Conditions: Fg y:  Fy = 0 X: Fx = 0  Fy = FN-Fg  Fx = Fapp-Fs  FN= Fg Fs= Fapp

  4. Example 2:- increasing applied force -fridge JUST begins to move -static friction is at MAX value +y +x FN Fapp Fs max Fg Force Conditions: X: Fx = 0 Y : FN= Fg  Fx = Fapp-Fs max Fs max= Fapp

  5. Example 3: -fridge accelerating forward. +y +x a FN FBD: Fk Fapp Fg Force Conditions: Y: FN= Fg X:  Fx = ma  Fx = Fapp-Fk Fapp–Fk=ma

  6. Finding Force of Friction: Depends on: 1) surface characteristics 2) the normal force pushing surfaces together Formulae: Fk= kFN FssFN Fs max =sFN Coefficient of static friction Coefficient of kinetic friction Starting Friction

  7. Typical coefficients: SurfacesSK 1.1 1.0 Rubber on dry concrete 0.1 0.03 Steel on ice 0.60 0.40 Steel on steel, dry 0.12 0.05 Steel on steel, greasy 0.22 0.18 Wood on dry snow Wood on wet snow 0.14 0.10

  8. Example 1: A child pulls a toy box (mass 15.0 kg) across a rough wooden floor using a rope inclined at 20.0 above the horizontal. The child applies a force of 100.0 N [W]. The coefficient of kinetic friction between the floor and the box is 0.250. Find: a) the normal force on the box b) the frictional force on the box c) the acceleration of the box. up, +y NOTE: The Normal force DOES NOT equal Fg!! FN = ? E, +x Fapp = 100.0 N Fappy Fk = ? Fappx Fg = ? Ans: a) 113 N b) 28.2 N [E] c) 4.38 m/s2 [W]

  9. Example 2: A child pulls a toboggan (mass 2.5 kg) up a snowy hill at a constant velocity as shown below. The coefficient of kinetic friction between the toboggan and snow is 0.15. Find the child’s applied force. FN FAPP NOTE: The Normal force DOES NOT equal Fg!! FK 10 Fg Ans: 7.9 N

  10. HW Question: Given ONLY a ruler, develop an experimental technique to determine the coefficient of static friction between your calculator and a textbook. HINT: Your textbook can be used as an inclined plane! Homework: Section 2.4 pg. 101 Practice Questions: 3,4, 5,6,7 pg. 101 Section Question 3 pg 106

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