1. Friction The force that opposes motion Depends on: Type of surface (material) Type of surface (material) Mass of object Mass of object

2. Friction! Proportional to Normal Force Proportional to Normal Force µ = coefficient of friction µ = coefficient of friction Friction Force: Friction Force: Ff = µF N Ff = Friction Force (N) FN = Normal Force (N) µ = coefficient of friction

3. Types of Friction Forces Static Static Resists the initial sliding motion between two surfaces that are in contact and at rest Resists the initial sliding motion between two surfaces that are in contact and at rest Kinetic Kinetic Force that opposes the movement of two surfaces that are in contact and are sliding over each other Force that opposes the movement of two surfaces that are in contact and are sliding over each other Sliding – two objects rubbing against each other Sliding – two objects rubbing against each other Rolling – between circular object and surface Rolling – between circular object and surface Fluid – interaction between fluid and solid Fluid – interaction between fluid and solid

4. Strength of Friction Weakest Friction Force Strongest Friction Force fluid rolling sliding static

5. Sample Problem A 24kg crate initially at rest on a horizontal floor requires a 75N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor. A 24kg crate initially at rest on a horizontal floor requires a 75N horizontal force to set it in motion. Find the coefficient of static friction between the crate and the floor.

6. Practice D 1. Once the create in Sample problem D is in motion, a horizontal force of 53N keeps the crate moving with a constant velocity. Find µk, the coefficient of kinetic friction, between the crate and the floor. 2. A 25kg chair initially at rest on a horizontal floor requires a 165N horizontal force to set it in motion. Once the chair is in motion, a 127 N horizontal force keeps it moving at a constant velocity A) find the coefficient of static friction between the chair and the floor B) find the coefficient of kinetic friction between the chair and the floor 3. A museum curator movies artifacts into place on various different display surfaces. Use the values in Table 2 to find F smax and Fk for the following situations: A) moving a 145kg aluminum sculpture across a horizontal steel platform B) pulling a 15kg steel sword across a horizontal steel shield C) pushing a 250kg wood bed on a horizontal wood floor D) sliding a 0.55 kg amulet on a horizontal glass display case