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4.3 Friction

Chapter 4 Force Ewen et al (2005). 4.3 Friction. Objectives: Identify components of friction. Analyze forces in one dimension. Friction. Friction is a force that resists the relative motion of two objects in contact with each other.

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4.3 Friction

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  1. Chapter 4 Force Ewen et al (2005) 4.3 Friction • Objectives: • Identify components of friction. • Analyze forces in one dimension.

  2. Friction • Friction is a force that resists the relative motion of two objects in contact with each other. • Caused by irregularities in the two surfaces sliding or rolling across each other. Figure 5.5 Friction resists motion of objects in contact with each other.

  3. Friction is both a necessity (good) and a hindrance (bad). • Experiments indicate the following general characteristics for frictional forces. • Friction is a force that always acts parallel to the surface in contact and opposite to the direction of motion. • Starting friction is greater than sliding friction. • Friction increases as the force between the surfaces increases.

  4. Figure 5.6 Friction increases as the force between the surfaces increases.

  5. Formal Description of Friction • Friction can be described with the following equation:whereFf = frictional forceFN = normal force (force perpendicular to the contact surface.) = coefficient of friction • The coefficient of friction () is the ratio between the frictional force (Ff)and the normal force (FN).

  6. Table 4.1 Coefficients of Friction ()

  7. How to reduce sliding friction: • Use smoother surfaces (but not too smooth). • Use lubrication to provide a thin film between surfaces. • Use Teflon to greatly reduce friction between surfaces when an oil lubricant is no desirable, such as electric motors. • Substitute rolling friction for sliding friction. Use wheels or bearings.

  8. Example 1 • A force of 170 N is needed to keep a 530-N wooden box sliding on a wooden floor. What is the coefficient of sliding friction?

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