2. Friction Friction is a force that opposes the motion, or tendency of motion, of an object. Friction is primarily caused by the chemical bonding of molecules at the surfaces of objects in contact (intermolecular forces which are electrical in nature)

3. 2 Types of “dry” Friction – Static friction exists between the surfaces of non-moving objects Opposes applied forces that would otherwise cause motion Maximum static friction refers to the most force that can be applied before the object starts to move – Kinetic friction Exists between the surfaces of objects when there is relative motion between the objects

4. Coefficient of Friction The coefficient of friction is the ratio of the force of friction to the normal force acting between two surfaces. Since this is a ratio of force to force, there are no units for the coefficient of friction This is an experimentally determined value for any two types of surface combinations The coefficient for static friction is generally larger than that of kinetic friction between surfaces.

5. Coefficient of Static Friction F s – Static friction force F n = Normal force

6. Coefficient of Kinetic Friction F k – Kinetic friction force F n – Normal force

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

8. Example – kinetic, constant speed The coefficient of friction between a 12 kg wooden crate and the floor is 0.32. How much force is needed to push this crate across the floor at a constant speed?

9. Example – accelerated motion A 5.0 kg box is pushed horizontally across the floor with a force of 25.0 N. If the coefficient of kinetic friction is 0.24, what is the acceleration of the box?

10. When the applied force is acting at an angle… Remember, a component of that force acts vertically and a component acts horizontally An upwards component will tend to separate surfaces reducing the normal force and thus reducing frictional force since f = μF N FaFa F a y = F a sinθ F a x = F a cosθ θ

11. When the applied force is acting at an angle… A downwards component will tend to squeeze surfaces together increasing the normal force and thus increasing the amount of frictional force since f = μF N When there is a vertical component, it is most likely that the normal force does NOT equal the weight

12. Example – applied force at an angle A 15 kg sled is dragged across the level snow at a constant speed by a force of 40.0 N that is applied 28° above the horizontal. Determine the coefficient of friction between the sled and snow.