1. 6 Applications of Newton’s Laws 6-1 Frictional Forces What is friction? To slide one such surface across another requires a force large enough to overcome the resistance of microscopic hills and valleys bumping together.

2. Kinetic Friction For an object moving : coefficient of kinetic friction; independent of area N: Normal force µk=µk= Ski on snow 0.05 Wood on leather 0.4 Rubber on concrete 0.8

3. Properties of kinetic frictional force Opposes motion in direction of surface Depends on smooth of surface Increase as force pressing surface increasing

4. How do we apply ? x y

5. Ex. Driving at 12 m/s slow on brake, m=1000kg, the coefficient µ k is 0.8, what is the acceleration? How long to stop?

6. Static Friction For a stationary object V=0 Object is static, until F>f s(max) object starts motion coefficient of static friction µs=µs= Ski on snow 0.1 Wood on leather 0.5 Rubber on concrete 1-4

7. Independent of the area of contact Static friction is not relative with the normal force, (direction and magnitude), except f s(max) Direction of f s is parallel to the surface of contact, and opposite to the direction the object would move if there were no friction, (opposite to the direction of the tendency of motion)

8. θ x y Change the tilt angle θ, let the static friction get the maximum value, but the box still rest, find the coefficient of static friction?