# Friction: Static and kinetic

##### Intros
###### Lessons
1. Introduction to friction
1. What causes friction?
2. What is kinetic friction?
3. What is static friction?
##### Examples
###### Lessons
1. Understanding kinetic vs. static friction

Joe wants to pull his 136 kg fridge away from the wall to clean behind it. When he pulls on the fridge with a horizontal force of 155 N, it doesn't move.

i. What is the force of friction that acts on the fridge? What type of friction is this?

ii. If the coefficient of friction between the fridge and floor is 0.200, what is the maximum amount of static friction that can act on the fridge?

iii. Joe gets Samantha to pull with an additional 155 N and the fridge begins to accelerate at 0.400 $m/s^{2}$. What is the coefficient of friction between the fridge and the ground? What type of friction is this?

1. Solving friction problems
1. A crate is pushed across the floor. The coefficient of friction between the crate and the floor is 0.240. If 167 N of applied force is required to keep the crate moving forwards at a constant velocity, find the mass of the crate.
2. A physics student presses a 0.700 kg textbook against a wall with a force of 30.0 N perpendicular to the wall, and the book does not slide down the wall.

i. What is the force of friction acting on the book? What kind of friction is this?

ii. The student slowly reduces their push. If the book begins to slide when the student pushes with 20.0 N, what is the coefficient of friction?