1. Friction is one of the most important opposing forces.
Remember…
Friction is one of the most important opposing forces.

2. Static friction…
The force of friction keeping an object in equilibrium even with a small applied force.

3. Kinetic friction…
Once an object begins to move, the static friction becomes known as kinetic friction
Kinetic means motion!!!

4. The types of materials in contact creates a certain amount of frictional force. This force is known as the coefficient of friction.
µk= Fk/ Fn
µk is coefficient of kinetic friction
Fk is the force causing motion
Fn is the normal force (mass x g)

5. µs= Fs,max/ Fn
µs is static coefficient of friction
You can calculate the frictional force if you know the µ and the normal force (mass x g)

6. Example 1:
A 24kg crate initially at rest on a flat surface requires 75N of force to set it in motion. What is the coefficient of static friction between the floor and crate?

7. Example 2:
Once the crate (from example 1) is moving it only requires 54N to keep it in constant motion. Calculate the kinetic coefficient of friction between the moving crate and the floor.

8. Overcoming a frictional situation…
Sometimes if you cannot get an object to move easily due to friction, size, location, etc., you may have to “alter” the situation. For example, apply wax to “slick” a floor, place something on wheels or tie a string in order to pull.

9. Example 3:
a student attaches a rope to a 44lb crate of books. If he pulls the rope with 90N of force at a 30 degree angle from the horizontal. If the coefficient of kinetic friction is 0.5, what is the forward acceleration of the box?

10. Example 4:
A 75 kg box slides down a 25 degree incline at an acceleration of 3.6 m/s2. Find the coefficient of kinetic friction between the box and the ramp.
Is there an applied force???